s 

45  .EG: 
BUL  24 


THE  GEOLOGY 

OF  THE 


SHAETEE  SILYEE  MINE  DISTEICT, 


PRESIDIO  COUNTY,  TEXAS. 


J.  A.  TTDDEN,  Ph.  I>.. 

Professor  of  Geology,  Augustana  College,  Rock  Island,  111.; 
Special  Assistant  Iowa  Geological  Survey. 


BULLETIN  OK  THE  UNIVERSITY  OF  TEXAS  NO.  24. 


- 


Entered  in  the  postoffice  at  Austin,  Texas,  as  second-class  mail  matter , 


AUSTIN: 

VON  BOECKMANN-JONE8  COMPANY,  STATE  PRINTERS, 
1904. 


I 


CORRECTION  FOR  TERLINGUA  TOPOGRAPHIC  SHEET. 

The  words  “True  North”  and  “ Magnetic  North”  on  the  diagram 
in  the  lower  left  margin  of  sheet  have  been  reversed  by  mistake. 
Correct  same  by  pasting  thereon  the  diagram  shown  on  this  slip  or 
by  transposing  the  words  “true”  and  “magnetic.” 


si* 


APPROXIMATE  MEAN 
DECLINATION  1902 


J 


299-104-6m. 

THE  UNIVERSITY  OF  TEXAS  MINERAL  SURVEY. 

BUI^ETIN  NO.  8-JUNK,  1904. 


THE  GEOLOGY 


OF  THE 

SHAETER  SILVER  MINE  DISTRICT, 

PRESIDIO  COUNTY,  TEXAS. 


BY 

J.  A.  UDDEN,  Ph.  D., 

Professor  of  Geology,  Augustana  College,  Bock  Island,  111.  ; 
Special  Assistant  Iowa  Geological  Survey. 


BULLETIN  OF  THE  UNIVERSITY  OF  TEXAS  NO.  24. 


Entered  in  the  postoffice  at  Austin,  Texas,  as  second-class  mail  matter. 


AUSTIN: 

VON  BOECKMANN- JONES  COMPANY,  STATE  PRINTERS, 
1904. 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/geologyofshafterOOudde 


~T~ 2>  i ^ 

. ? 


LETTER  OF  TRANSMITTAL. 


Hon.  Wm.  L.  Prather , President , The  University  of  Texas. 

Dear  Sir  : I beg  herewith  to  transmit  to  yon  a report  on  the  Geology 
of  the  Shafter  Silver  Mine  District,  Presidio  county,  Texas,  prepared 
by  Prof.  J.  A.  Udden,  Augustana  College,  Rock  Island,  111.  Professor 
Udden  spent  several  months  in  the  field  for  us  last  year  and  the  result 
of  his  work  appears  in  this  Bulletin. 

At  Shafter,  Presidio  county,  is  situated  the  only  successful  silver  mine 
in  the  State.  It  has  been  in  continuous  operation  for  nearly  twenty  years 
and  has  yielded  to  its  owners  very  profitable  returns. 

It  was  thought  that  a special  report  on  this  district  would  be  of  value 
to  the  further  development  of  the  public  school  lands  in  that  part  of  the 
Sate. 

Very  respectfully, 

William  B.  Phillips, 

June,  1904.  Director. 


ANNOUNCEMENT. 


The  University  of  Texas  Mineral  Survey  is  prepared  to  undertake  all 
kinds  of  chemical  analyses,  assays,  investigations  of  technical  processes, 
etc.  It  has  a well-equipped  laboratory,  furnished  with  modern  scientific 
appliances,  and  can  make  returns  promptly.  Prices  on  application. 

Address  all  communications  to 

Dr.  Wm.  B.  Phillips, 

Director,  University  of  Texas  Mineral  Survey, 
Austin,  Texas. 


CONTENTS 


Page. 

Location  and  limitation  of  the  District 7 

Topography 7 

The  Chinati  mountains 7 

The  uplands  of  Cibolo  and  Cienega  creeks 8 

The  gravel  flats  of  the  Rio  Grande 9 

Drainage 9 

Stratigraphy. ' 9 

The  sedimentary  rocks 11 

The  Chinati  series 11 

The  Cieneguita  beds 13 

The  Alta  beds 15 

The  dark  shales 15 

The  yellow  sand # 18 

The  Cibolo  limestones % 18 

The  transition  beds 18 

The  lower  brecciated  zone 20 

The  zone  of  sponge  spicules 21 

The  thin-bedded  zone — 21 

The  yellow  limestone 22 

Geographic  conditions 23 

Correlations . 23 

The  Cretaceous  rocks 25 

The  Presidio  beds 25 

Sections 27  to  30 

General  character 29 

Fossils 30 

Correlation 30 

The  Shatter  beds 30 

Exposures 31-38 

Fossils  of  the  Shatter  beds 37 

Correlations  38 

Geographical  conditions 38 

The  Edwards  limestone 39 

The  Del  Rio  clay  [?] 39 

The  Buda  limestone  [?] 40 

The  land  drift 40 

The  Rio  Grande  drift 41 

The  igneous  rocks 42 

Laccolitic  bosses 42 

Dikes  and  intrusive  sheets. . . 43 

Lavas  and  tuffs .. 43 

Structures 44 

Chinati  uplift 44 

The  Morita  uplift 46 

Dips 46 


6 


Contents 


Page. 

Tangential  faults 48 

Radiating  faults , 50 

Secondary  changes  in  some  rocks 52 

Igneous  metamorphism 52 

Changes  effected  by  ground  moisture 52 

Mineral  deposits 54 

Lodes 54 

Chimneys 54 

Contacts 54 

Fissures 55 

Fault-fissures 57 

Fissured  zones 59 

Summary  of  geological  events 59 

Acknowledgments. 60 


GEOLOGY  OF  THE  SHAFTER  SILVER  MINE  DISTRICT, 
PRESIDIO  COUNTY,  TEXAS. 

BY  J.  A.  UDDEN,  PH.  D. 


LOCATION  AND  LIMITATION  OF  THE  DISTRICT. 

For  purposes  of  discussion  in  the  present  paper  we  will  designate 
as  the  Shatter  Mine  District  an  area  in  Presidio  county  about  twenty 
miles  square,  with  the  silver  mine  at  Shatter  near  the  centre.  Thus 
defined,  this  district  includes  the  greater  part  of  the  Chinati  mountains 
to  the  northwest,  it  reaches  out  almost  across  the  flats  of  the  Rio  Grande 
to  the  southwest,  and  extends  to  the  Cienega  mountain  on  the  east. 
Geologically  it  may  be  said  to  approximate  a unit,  the  Chinati  uplift 
being  the  dominant  feature  which  controls  its  topography  as  well  as  its 
geological  structure.  The  district  is  40  to  50  miles  south  a little  west 
from  Marfa,  a station  on  the  Southern  Pacific  Railway,  196  miles 
southeast  of  El  Paso. 

TOPOGRAPHY. 

The  average  altitude  of  the  Shatter  Mine  district  is  about  4500  feet 
above  sea  level.  The  highest  point  on  the  Chinati  mountains  rises  to 
7730  feet  and  the  elevation  of  the  nearest  point  on  the  Rio  Grande  is 
about  2575  feet.  Thus  the  extreme  amount  of  relief  in  the  district 
is  nearly  5155  feet. 

Topographically  the  area  may  be  roughly  divided  into  three  parts: 
the  Chinati  mountains,  the  uplands  of  Cibolo  and  Cienega  creeks,  and 
the  flats  of  the  Rio  Grande. 

The  Chinati  mountains  occupy  an  area  which  is  elliptical  in  outline, 
with  the  long  axis  extending  from  northwest  to  southeast,  a distance  of 
about  fourteen  miles.  They  continue  beyond  the  boundaries  of  the  dis- 
trict by  an  extension  to  the  north  of  some  lower  peaks  and  ridges.  The 
north  end  of  the  main  mountain  is  highest  and  is  separated  from  the 
south  end  by  a sag  in  the  crest  caused  by  an  eastward  encroachment  of 
the  San  Antonio  Canon.  This  part  of  the  mountain  forms  a dissected 
dome  about  five  miles  in  diameter.  The  south  end  has  the  greater  areal 
extent,  being  about  eight  miles  long  and  five  miles  wide.  It  is,  strictly 
speaking,  a terraced  and  dissected  mesa,  which  rises  about  2500  feet 
above  the  surrounding  country.  It  is  built  up  of  several  superimposed 
flows  of  igneous  rock,  and  all  around  the  mountain  these  form  suc- 
cessive steps  of  vertical  cliffs  separated  by  intervening  talus  slopes. 
The  relief  of  the  mesa  is  still  more  diversified  by  incisions  of  canons 
from  all  directions.  On  the  north  side  these  canons  are  narrow  and 
short,  seldom  extending  more  than  a mile  back  from  the  periphery.  On 
the  south  side  the  drainage  comes  down  chiefly  through  three  wide 
canons,  which  might  rather  be  described  as  irregular  amphitheaters. 
These  reach  back  to  nearly  three-fourths  of  the  distance  across  the 
mountain.  Several  circumstances  have  contributed  to  the  greater 
efficiency  of  the  erosion  on  the  south  side  as  compared  with  that  on  the 


8 


Geology  of  the  Shafter 


north.  Draining  directly  into  the  Rio  Grande,  the  streams  on  this  side 
have  a much  steeper  gradient  than  those  on  the  north  side,  for  the  lat- 
ter join  the  Cibolo  creek  at  an  elevation  of  about  1500  feet  above  the 
river.  The  disintegration  of  the  south  side  of  the  mesa  is  likewise  more 
rapid,  owing  to  greater  extremes  of  changing  temperature  on  the  sunny 
side.  Probably  also  the  rainfall  of  this  region  is  slightly  greater  on 
that  side  of  the  mountain.  The  largest  of  these  canons,  or  amphithea- 
ters, is  more  than  1500  feet  deep  and  is  locally  known  as  the  Black 
Canon.  The  top  of  the  east  part  of  the  Chinati  mountains  is  crowned 
by  some  small  peaks,  which  are- the  last  remnants  of  a lava  flow  which, 
like  the  rest,  was  formerly  continuous  across  the  entire  mesa  and  far 
beyond. 

The  Uplands  of  Cibolo  and  Cienega  Creeks.  Looking  away  from  the 
Chinati  mountains  to  the  east  we  find  an  upland  of  quite  varied  relief. 
A succession  of  alluvial  fans  and  cones  extend  out  from  the  base  of  the 
mountain  in  an  almost  continuous  belt  of  varying  width.  The  greater 
part  of  this  belt  is  dissected  considerably  by  the  present  drainage,  but 
the  old  slopes  are  still  to  be  seen  more  or  less  distinctly.  Beyond  this 
belt  to  the  south  and  in  part  to  the  east  lie  a range  of  foothills  formed 
by  the  most  indurated  members  of  the  sedimentary  rocks  of  the  region. 
These  ridges  are  highest  near  the  south  end  of  the  mountain  and  are 
there  separated  from  the  latter  by  a valley  not  much  exceeding  a mile 
in  width.  Turning  to  the  northeast  and  then  to  the  north  the  foot  hills 
flatten  out  and,  at  the  same  time,  diverge  away  from  the  mountains. 
Outside  of  these  hills  there  is  a belt  of  varying  width,  approximately 
co-extensive  with  the  remaining  outcrops  of  the  sedimentary  rocks, 
where  low  ridges,  frequently  crossed  by  small  drainage  valleys,  run  par- 
allel with  the  foot  hills  around  the  mountain  on  the  south  and  east  side. 
These  ridges  have  long  and  low  slopes  on  the  outer  side,  in  the  direc- 
tion of  the  dip,  and  shorter  and  more  abrupt  declivities  and  even  verti- 
cal scarps  on  the  inner  side.  Over  a part  of  the  area  of  these  sediments 
the  ridge  structure  is  somewhat  subdued  or  even  entirely  concealed  by 
a covering  of  stream  drift-.  This  topography  in  part  occupies  a triangle 
whose  corners  lie,  one  near  Spencer’s  spring,  another  at  the  Cibolo 
Ranch,  and  the  third  at  a point  a mile  below  the  junction  of  the  Morita 
and  Cibolo  creeks.  In  part  it  extends  as  a belt  no  more  than  a mile 
wide,  first  to  the  north  from  the  Cibolo  ranch  and  then  to  the  northwest 
as  far  north  as  the  Shatter  area  extends.  The  remaining  part  of  this 
topographic  division  consists  of  lava-capped  mesas  and  peaks  wTith  inter- 
vening stretches  of  flat  drift  plains.  The  higher  peaks  exhibit  the  same 
terraced  slopes  as  seen  in  the  Chinati  mountains.  The  highest  extend 
in  a line  from  north  to  south,  east  of  Shatter,  rising  1200  feet  above 
Cibolo  creek.  The  Aguja  (needle),  which  stands  on  the  flat  top  of  a 
wide  mesa  between  the  upper  branches  of  the  Cibolo,  is  the  most  sym- 
metrical of  all  the  peaks  in  this  region.  Three  other  features  add 
variety  to  the  topography  of  these  uplands : some  small  areas  of  granite 
to  the  north  weather  into  rounded  irregular  knobs  and  hills  of  small 
size;  a crescent  shaped  basin  is  inclosed  by  highly  tilted  sediments  along 
the  lower  course  of  Morita  creek;  and  east  of  this  the  west  half  of  the 
low  dome  of  the  Cienega  mountains  rises  to  a flat  summit  on  the  east 
boundary  of  the  district. 

The  Gravel  Flats  of  the  Rio  Grande.  These  occupy  some  twenty 


Silver  Mine  District. 


9 


square  miles  in  the  southwest.  They  have  an  even  surface,  almost  their 
only  relief  consisting  of  a few  valleys,  or  arroyos,  that  run  in  a general 
direction  from  northeast  to  southwest.  The  surface  of  the  plain  rises 
from  2600  feet  above  the  sea  at  the  southwest,  where  the  Rio  Grande 
is  only  about  a mile  distant,  to  3500  feet  and  more  on  the  north,  ten 
miles  away  from  the  river.  This  gives  it.  a slope  of  about  90  feet  to 
the  mile.  The  upper  reaches  of  these  flats  are  evidently  continuous 
with  the  drift  plains  of  the  higher  country  and  merge  into  these  imper- 
ceptibly. 

DRAINAGE. 

The  present  drainage  is  consequent  to  the  configuration  of  an  older 
land.  The  general  courses  of  the  two  main  streams,  the  Cibolo  and  the 
Cienega,  were  evidently  determined  by  the  surface  of  the  old  lava 
flows  which  once  covered  the  district.  There  are  indications  that  the 
Sierra  Alta  branch  of  the  Cibolo  creek  may  have  taken  a part  of  its 
drainage  area  from  the  Cienega  system.  Usually  the  major  features  of 
the  drainage  do  not  show  much  adjustment  to  rock  structure.  On  the 
surface  of  the  sedimentary  rocks  the  streams  are  clearly  superimposed. 
Adjustment  to  the  structure  of  these  rocks  appears,  however,  in  many 
minor  features,  as  in  the  location  of  the  ultimate  ramifications  of  the 
systems  and  in  the  occasional  widening  of  the  larger  drainage  valleys 
when  crossing  or  following  strata  that  are  easily  disintegrated.  In 
brief,  we  may  say  that  the  larger  streams  are  superimposed  while  the 
smaller  arroyos  are  mostly  subsequent. 

Though  there  are  no  undrained  basins,  the  surface  run-off  makes  a 
very  small  percent  of  the  rainfall.  This  is  due  not  only  to  the  rapid 
evaporation  in  a warm  and  dry  climate,  but  also  to  an  efficient  under- 
ground drainage  and  to  the  open  texture  of  the  surface  deposits.  It 
is  only  after  heavy  rains  that  the  channel  of  Cibolo  creek  carries  a con- 
tinuous stream.  During  the  dry  season  its  water  appears  on  the  sur- 
face only  in  a few  places  above  rocky  sills.  Elsewhere  its  water  sinks 
into  the  bed  of  sand  and  gravel.  Some  of  the  ground  water  in  this 
basin  descends  far  below  the  level  of  its  main  outlet  and  must  be  car- 
ried away  by  some  other  route.  The  shaft  of  the  Presidio  mine  is  per- 
fectly dry  300  feet  below  the  bottom  of  Cibolo  creek.  This  under- 
ground drainage  no  doubt  follows  some  of  the  many  joints,  faults  and 
disintegrated  dikes  that  traverse  the  sedimentary  rocks  in  the  district. 

STRATIGRAPHY. 

Sedimentary  rocks  underlie  the  greater  part  of  the  entire  district. 
The  oldest  of  these  belong  to  the  Paleozoic  group  and  comprise  a single 
great  sedimentary  cycle.  These  will  be  designated  as  the  Chinati  series. 
They  are  overlaid  by  sediments  of  the  lower  Cretaceous  age,  which  again 
are  partly  covered  by  the  land  drift.  The  greatest  total  thickness  of 
all  the  sediments  approximates  7,500  feet.  They  will  here  be  described 
in  the  order  from  below  upward. 

The  sediments  have  been  displaced  and  uplifted  by  deep  intrusives, 
which  lie  under  them,  and  which  have  caused  the  exposure  of  their  tilted 
edges  around  the  uplifts.  On  top  of  all  of  this  we  find  a series  of  extru- 
sive volcanics  which  have  concealed  most  of  the  sediments  and  probably 
also  much  of  the  deep  intrusives.  The  table  below  exhibits  these  gen- 
eral relations  of  all  the  rocks  in  the  district.  (See  also  Fig.  1.) 


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Silver  Mine  District. 


11 


TABLE  SHOWING  THE  GENERAL  ARRANGEMENT  OF  THE  ROCKS  IN  THE 
SHAFTER  MINE  DISTRICT. 


Group. 

System. 

Series. 

Formation. 

Cenozoic. 

Pleistocene. 

Land  and  stream  drift. 

Tertiary. 

Late  tertiary. 

Lavas,  and  dikes. 

Early  tertiary. 

Deep  intrusives. 

Mesozoic. 

Cretaceous. 

Lower  cretaceous. 

Buda  limestone  (?). 

Del  Rio  clay  (?). 
Edwards  limestone. 
Shafter  beds. 
Presidio  beds. 

Paleozoic. 

Permian  (?)  and 
Carboniferous. 

Chinati  series. 

Cibolo  beds. 

Alta  beds. 
Cieneguita  beds. 

THE  SEDIMENTARY  ROCKS. 

The  Chinati  Series. 

The  Paleozoic  rocks  which  are  found  within  the  Shatter  Area  form 
one  successive  series,  changing  from  conglomeritic  rocks  below  through 
arenaceous  beds  and  clays  to  limestones  above.  They  appear  to  belong 
to  the  Upper  Carboniferous  age  and  possibly  extend  up  into  the  Per- 
mian. They  occupy  a semicircular  belt  around  the  Chinati  mountains 
to  the  northeast,  east,  and  southeast.  This  belt  varies  from  one  to  two 
miles  in  width  and  averages  a little  less  than  a mile  and  a half.  Near 
the  silver  mine  a cusp  extends  a short  distance  away  from  the  main  belt 
to  the  south.  At  the  north  it  makes  a short  and  sharp  detour  to  the 
northeast  around  a small  granitic  area.  To  the  southwest  it  disap- 
pears under  the  gravels  of  the  Eio  Grande  flats.  An  isolated  outcrop 
was  also  found  about  six  miles  to  the  south-southeast  of  Shafter,  and 
this  exposed  the  lower  members.  This  is  in  the  basin  of  Morita  creek, 
and  the  rocks  occur  in  a narrow  belt  a mile  and  a half  long,  following 
this  stream  from  northeast  to  southwest. 

For  convenience  in  description  the  series  will  be  considered  in  sev- 
eral divisions,  beginning  below,  and  taking  them  in  ascending  order. 
These  divisions  will  be  designated  as  indicated  in  the  following  table: 
(See  also  Fig.  2.) 


Fig.  2.— General  Section  of  the  Chinati  Series,  about  5 miles  north  of  Shatter.  From  east  to  west.  A,  Presidio  beds;  B,  Cibolo  limestone;  C,  Transition  beds; 

D,  Alta  sandstone;  E,  Alta  shales;  F,  Oienequita  beds;  G,  Granite  near  Ojo  Bonito. 


Silver  Mine  District. 


13 


Section  of  the  Chinati  Series. 


The  Cibolo  Beds. 

8.  The  Yellow  Limestone 

8.  The  Thin-bedded  Zone  

6.  The  Zone  of  Sponge  Spicules 
5.  The  Lower  Brecciated  Zone. 
4.  The  Transition  Beds  

The  Alta  Beds. 

3.  The  Yellow  Sand  

2.  The  Dark  Shales  

The  Cieneguita  Beds. 

1.  Basal  deposits  


Thickness 
in  feet. 


650 

470 

85 

133 

100 


1500 

2000 

1000 


The  Cieneguita  Beds. 

The  lowest  of  the  Carboniferous  sediments  consist  of  a series  of  dark 
or  almost  black  shales,  alternating  with  heavy  lenticular  masses  of  mor- 
tar rocks1,  conglomerates,  dark  limestones  and  of  mixtures  of  these 
materials.  Some  of  these  masses  are  upward  of  thirty  feet  in  thickness 
and  extend  for  more  than  a mile  horizontally.  The  shale  constitutes  the 
greater  part  of  the  division.  In  places  it  contains  some  layers  of  black 
chert  and  in  the  lower  part  some  greenish  [glauconitic?]  seams  were 
noted,  and  also  some  flattened  round  concretions  containing  a dark 
grayish  powdery  material.  Farthest  down,  close  to  a granite  boss, 
against  which  this  shale  lies,  four  granite  or  gneiss  boulders^  consid- 
erably decayed,  were  found  imbedded  in  it.  They  ranged  from  two  to 
four  feet  in  diameter,  and  were  considerably  more  micaceous  than  the 
intrusive  granite  which  occurs  close  by,  and  apparently  different  from 
this  in  texture.  At  the  same  place  one  seam  showed  an  imprint  and 
some  carbonaceous  material  of  the  flattened  stem  of  a plant.  The  indu- 
rated masses  range  through  all  possible  gradations  of  conglomerates, 
sandstones,  limestones,  shell  breccias,  and  mortar  rocks.  The  latter  are 
the  most  common.  Occasionally  they  have  been  crushed  and  re-ce- 
mented, forming  crush-breccias.  Their  character  can  best  be  presented 
in  some  descriptions  of  individual  specimens. 

A sample  of  the  dark  limestone  was  seen  to  be  a breccia  of  shell  frag- 
ments, the  largest  of  which  were  about  1-25  of  an  inch  (one  mm.)*  in 
thickness,  and  exhibited  a finely  reticulated  structure  in  cross  section. 
With  these  were  smaller  fragments  of  shells,  joints  of  crinoid  stems, 
occasional  fusulinas,  minute  calcareous  spines  and  occasional  grains  of 
clear  quartz  mostly  angular.  All  of  this  was  imbedded  in  a compact 
calcareous  paste. 

Another  specimen  was  a gray  mortar  rock  consisting  of  sharply  angu- 
lar quartz  fragments  imbedded  in  a calcareous  matrix  of  fine  texture. 

xThis  term  is  used  to  designate  an  unaltered  clastic  rock  consisting  of  an  indu. 
rated  mixture  of  calcareous  mud  and  siliceous  fragments  of  variable  sizes  and 
degrees  of  wear, 

aOne  inch=25  millimetres,  about. 


14 


Geology  of  the  Shafter 


The  quartz  fragments  vary  in  size  from  pebbles  to  particles  of  minute 
size,  being  very  imperfectly  sorted.  Some  have  clearly  been  fractured 
in  the  rock,  and  the  broken  pieces  lie  close  together.  Most  of  the  quartz 
is  clear,  some  is  opalescent  white,  and  some  dark.  Mica  scales  were 
also  observed.  A single  pebble  was  seen  to  be  rounded.  In  some  other 
conglomerate  ledges  water-worn  quartz  pebbles  are  very  common. 

Still  another  ledge  is  a conglomerate,  for  the  most  part  calcareous,  or 
breccia  of  dark  gray  color.  The  body  of  this  consists  of  a calcareous 
paste,  in  which  unassorted  angular  quartz  grains  lie  imbedded. 
Occasionally  quartz  fragments  are  fractured  and  the  broken  pieces  of 
the  grains  lie  close  together,  the  intervening  fissure  being  filled  with  a 
calcareous  paste.  Most  of  the  quartz  is  clear,  some  whitish.  In  this 
mass  lie  larger  pebbles  of  quartz  and  of  limestone,  also  now  and  then 
some  mica  scales,  fragments  of  feldspar,  and  pieces  of  crinoid  stems. 
The  limestone  pebbles  are  mostly  of  a rock  entirely  different  from  that 
in  any  of  the  ledges  in  this  part  of  the  section.  It  is  white,  very  com- 
pact in  texture,  free  from  quartz,  and  obscurely  laminated.  These  peb- 
bles range  in  sizes  up  to  three  inches  in  diameter.  Many  are  plainly 
faceted  in  a manner  similar  to  the  faceting  of  small  boulders  and  peb- 
bles in  glacial  deposits.  These  facets  vary  in  number  from  two  to  five 
on  the  same  pebble,  they  are  oriented  at  angles  from  30°  to  120°,  and 
are  separated  by  rounded  edges. 

The  largest  of  the  quartz  pebbles  in  the  conglomeritic  ledges  of  this 
division  average  an  inch  in  diameter  and  are  usually  well  water  worn. 
Some  consist  of  white,  and  some  of  dark  vein  quartz.  With  these  a 
few  were  seen  which  consisted  of  a sort  of  graphic  granite  or  a peg- 
matite. The  feldspar  in  this  rock  is  in  thin  crystals,  oriented  so  as 
to  give  simultaneous  reflections,  when  turned  in  the  hand.  Most  of 
the  calcareous  ledges  contain  angular  quartz  grains  as  an  original  ingre- 
dient, and  most  of  them  also  contain  some  fossil  remains.  The  pres- 
ence of  some  large  boulders  in  the  lower  layers  and  the  nature  of  the 
conglomerate  with  the  small  faceted  cobbles  of  foreign  rock,  suggests 
transportation  <and  sedimentation  by  icebergs. 

The  following  fossils  were  found  in  the  uppermost  conglomeritic 
ledge  of  these  beds : 

Fusulina  cylindrica  Fischer. 

Stems  of  crinoids. 

Fewestella  binodata  Condra. 

Fistulipora  carbonaria  Ulr. 

Meekopora  prosseri  Ulr. 

Stenopora  heteropora  Condra. 

Seminula,  sp. 

Brachiopod  shells,  in  fragments. 

From  another  mortar  ledge,  which  lies  some  300  feet  lower  down, 
the  following  fossils  were  taken: 

Syringopora  multattennuata  McChesney. 

Lophophyllum  proliferum  McChesney. 

Crinoid  head,  and  stems. 

• Dielasma  bovidens  Morton. 

Productus  semireticulatus  Martin. 

Pugnax,  sp. 


Silver  Mine  District. 


15 


Reticularia  perplexa  McChesney  [ ?] 

Seminula  argentea  Shepard. 

Spirifer  earner atus  Morton. 

Macrocheilus  anguliferus  White  [?] 

Pleurotomaria,  sp. 

In  other  ledges,  mostly  between  these  two,  were  noted: 

Fusulina  cylindrica  Fischer. 

Syringopora  multattennuata  McChesney. 

Chonetes,  sp. 

Productus,  sp. 

Seminula  ar gen  tea  Shepard. 

Spirifer  cameratus  Morton. 

Straparollns,  sp. 

Macrocheilus,  sp. 

Dentalium,  sp.  (with  fine  longitudinal  flutings),  frequent. 

A fragment  of  a pelecypod. 

The  outcrops  of  this  division  lie  between  Cibolo  creek  and  some  of 
its  tributaries,  east  and  southeast  of  the  Ojo  Bonito  and  the  Cieneguita 
ranches.  A continuous  thickness  of  1000  feet  was  measured,  but  it  was 
evident  that  this  did  not  include  all,  and  possibly  not  one  half  of  its 
entire  development.  A continuous  section  of  the  entire  division  could 
not  be  found,  but  the  character  of  the  different  ledges  leaves  no  doubt 
that  they  belong  together. 

The  Alta  Beds. 

Above  the  conglomerate-bearing  beds  there  are  some  3500  feet  or  more 
of  perfectly  sorted  arenaceous  and  argillaceous  beds,  almost  destitute  of 
calcareous  material  and  of  fossil  remains.  These  make  two  divisions, 
some  dark  shaly  beds  below,  and  some  more  light  colored  and  sandy  beds 
above.  Each  is  best  described  separately. 

The  Dark  Shales.  This  bed  consists  of  a mixture  of  silt,  clay,  and 
some  sand.  Its  bedding  planes  are  straight  and  very  sharply  marked. 
Throughout  there  are  a number  of  laminae  of  coarser  and  more  purely 
sandy  material,  varying  in  thickness  from  a half  inch  to  one  foot.  The 
thinnest  are  usually  less  than  six  inches  apart,  while  such  as  exceed 
two  inches  in  thickness  are  seldom  separated  by  more  than  two  feet  of 
the  shale.  The  latter  are  sometimes  fissile  along  straight  horizontal 
seams,  where  mica  scales  lie  imbedded.  Calcareous  material  frequently 
fills  the  spaces  between  the  grains  in  these  layers,  evidently  a secondary 
deposit  from  infiltration.  Calcareous  and  ochreous  seams  also  occur, 
apparently  wholly  the  result  of  precipitation  from  percolating  ground- 
water.  The  sandy  layers  consist  of  angular  but  well  assorted  grains, 
the  bulk  of  which  range  from  \ to  -J  mm.  in  diameter.  The  body  of 
the  dark  shale  itself  is  of  the  same  nature,  only  finer,  and  less  perfectly 
sorted,  the  bulk  of  the  grains  ranging  from  \ to  ^ mm.  in  diameter. 
Usually  it  is  also  very  slightly  calcareous. 

It  may  be  that  a small  ledge  of  limestone,  noted  on  the  north  side 
of  Sierra  Alta  hill,*  belongs  in  the  lower  part  of  these  shales.  This 

*This  hill  lies  on  the  east  side  of  the  Sierra  Alta  creek,  about  three  miles  above 
its  junction  with  the  Cibolo  creek,  and  about  six  miles  north  of  Shafter. 


16 


Geology  of  tile  Shafter 


ledge  contained : Productus  [near  Cora] , a Chonetes  and  a Pleuroto- 
maria.  Otherwise  no  fossils  were  noted  in  these  sediments  other  than 
some  tracks  of  various  sizes  and  some  other  impressions  on  the  upper 
surface  of  some  of  the  sandy  layers.  These  run  in  curves,  in  all 
instances  seen.  Some  consist  of  a single  series  of  crescent-shaped 
impressions  lying  transverse  to  the  axis  of  the  path,  and  some  are  made 
up  of  a double  series  of  sigmoid  indentations  having  the  same  position. 
The  other  structures  may  or  may  not  be  of  an  organic  origin.  They 
are  flow  structures  of  the  kind  which  have  been  described  as  Spirophy- 
ton  in  other  regions.  There  are  also  some  tracks  consisting  of  shallow, 
smooth  linear  grooves,  resembling  what  are  commonly  known  as  fucoid 
marks.  (See  Fig.  3.) 


Fig.  3.— Tracks  of  Trilobites  (?)  in  sandy  seams  in  the  Alta  beds- 


18 


Geology  of  the  Shafteb 


The  thickness  of  these  shales  approximates  2000  feet. 

The  Yellow  Sand.  The  dark  shales  are  succeeded  above  by  a sand- 
stone of  tine  texture.  No  exposures  appear  for  a short  distance  where 
the  change  between  this  and  the  dark  shales  and  the  sandstone  occurs. 
The  rock  is  soft,  occasionally  almost  crumbling,  but  it  contains  some 
indurated  ledges  measuring  a few  feet  each.  Some  of  the  strata  of 
finer  texture  are  bluish  gray,  and  some  of  the  coarser  ledges  are  gently 
cross-bedded.  The  body  of  this  sandstone  is  a coarse  silt,  composed  of 
well  assorted  quartz  grains,  for  the  most  part  angular,  the  bulk  of  which 
range  from  J to  mm.  in  diameter.  In  the  coarser  strata  the  sort- 
ing is  still  more  perfect  and  there  is  mainly  only  a smaller  admixture  of 
grains  below  g1  mm.  in  diameter.  The  coarse  layers  are  most  abun- 
dant above.  Excepting  a few  track-like  imprints  in  the  fine  textured 
bluish  layers,  no  organic  vestiges  were  noted.  Thickness:  1550  feet. 

The  Dark  Shale  and  the  Yellow  Sand  appear  in  an  arroyo,  which 
runs  on  the  south  side  of  the  lava-capped  hill  known  as  the  Sierra  Alta. 
They  dip  at  high  angles,  from  20°  to  48°,  to  the  east,  and  the  thickness 
given  was  obtained  from  measurements  across  the  strike.  The  strike 
is  a little  west  of  north. 

The  Cibolo  Limestones. 

A series  of  limestones  makes  up  the  last  of  the  paleozoic  sediments  in 
this  area.  These  limestones  differ  from  those  in  the  lower  part  of  the 
series  in  several  respects.  They  contain  no  large  quartz  pebbles,  though 
there  is  sometimes  a little  sand.  They  mostly  have  a lighter  color. 
Fusulinas  occur  in  greater  abundance,  and  they  have  a much  greater 
development.  There  are  several  somewhat  different  horizons,  and  these 
are  best  described  separately.  They  are  most  favorably  exposed  in  the 
east  bluff  of  the  Sierra  Alta  creek,  which  comes  into  Cibolo  creek  from 
the  north,  about  a half  mile  below  the  Cibolo  ranch,  and  three  miles 
north  of  Shafter.  The  entire  depth  of  this  division  is  not  far  from 
1450  feet. 

At  the  locality  above  referred  to  the  different  horizons,  in  ascending 
order,  are  as  follows : 

1.  Transition  Beds. — Upward  the  Alta  sandstones  give  place  to 
a gray  marly  shale,  which  contains  some  lenticular  ledges  of  organic 
and  siliceous  sand.  One  of  the  lowermost  of  these  measured  eighteen 
feet  in  thickness  and  consisted  of  an  almost  pure  quartz  sand  below  and 
of  an  organic  sand  above,  which  was  composed  almost  wholly  of  worn 
organic  fragments.  In  this  division,  and  mostly  in  this  arenaceous  len- 
ticular mass,  the  following  fossils  were  noted : 


Fig.,4.  —1.  Stem  of  a Orinoid;  2,  3 and  4,  Siliceous  rock  containing  spicules  of  sponges.  The  Crinoid  stem  is  from  the  base  of  the  lower  brecciated 
zone.  The  others  are  from  the  zone  of  sponge  spicules.  All  from  the  Oibolo  beds. 


20 


Geology  of  the  Sh  after 


Fusulina  cylindrica  Fischer. 

Archseocidaris,  sp.  [spines  and  plates]. 

Crinoids  [stems  and  plates]. 

Meekopora  prosseri  Ulr. 

Stenopora  heteropora  Condra. 

Fistulipora,  sp. 

Meekella  occidentalis  Newb. 

M.  striutocostata  [Cox]. 

Productus  semireticulatus  [Martin]. 

Spirifer  cameratus  [Morton]. 

Spirifer,  nndet. 

Straparollus  [two  large  species,  two  and  three  inches  wide]. 

Bellerophon  crassus  Mk.  & W. 

Conulccria  crustula  White. 

Dentalium,  sp. 

Myalina,  sp. 

These  marly  beds  are  seen  for  a distance  of  nearly  three  miles  in  the 
foot  of  the  ridge  which  runs  south  from  the  Sierra  Alta  hill  and  on  the 
east  side  of  a creek,  usually  known  by  the  same  name.  Including  the 
fossil-bearing  lenses,  they  measure  about  100  feet. 

2.  Lower  Brecciated  Zone.  This  consists  of  a grayish  white  lime- 
stone in  heavy  ledges,  sometimes  many  feet  in  thickness,  and  often 
thoroughly  brecciated.  In  places  the  breccia  is  very  coarse,  containing 
blocks  six  feet  in  diameter.  At  one  point  the  middle  part  of  these  beds 
was  not  brecciated.  The  rock  is  sometimes  crowded  with  minute  casts 
of  foraminifera,  more  or  less  distinct  in  outline.  These  lie  in  a copious 
compact  matrix.  In  one  specimen  some  round  grains  were  seen  to  have 
a white  oolitic  coating.  The  rock  is  more  or  less  fossiliferous  through- 
out. The  basal  part  contains  some  very  large  crinoid  stems,  the  largest 
seen  measuring  inches  in  diameter,  and  with  these  were  some 
crinoid  heads.  The  following  fossils  were  observed,  mainly  in  the 
lowermost  ledges : 

Fusulina  cylindrica  Fischer. 

Crinoids,  heads,  three  or  four  species. 

Archseocidaris,  sp. 

Fenestella  gracilis  Condra. 

Fistulipora  carbonaria  Ulr. 

Stenopora  heteropora  Condra. 

Polypora  bassleri  Condra. 

Meekopora  prosseri  Ulr. 

Spirifer  cameratus  Morton. 

S.  kentuckensis  Shum  [?]. 

Spirifer,  sp.  [very  broad  form,  four  or  five  inches,  with  high  area]. 

Meekella  striatocostata  Cox. 

Capulus  parvus  Swallow  [ ?] . 

Productus  longispinus  Sow. 

P.  semireticulatus  Martin. 

Chonetes,  sp. 

Myalina,  sp. 

Straparollus,  sp. 

Bellerophon,  sp. 


Silver  Mine  District. 


21 


Waagenoceras,  near  cumminisi  White. 

The  thickness  of  this  member  is  about  133  feet. 

The  Zone  of  Sponge  Spicules. — This  consists  of  more  thin-bedded 
rock,  running  in  even  and  straight  ledges  from  six  inches  to  one  foot 
in  thickness.  Below,,  these  are  of  pure  limestone  but  above  they  are 
siliceous  and  run  into  pure  sandstone.  Some  ledges  of  finer  sediments 
split  into  very  thin  straight  and  smooth  laminae  from  weathering.  The 
lower  ledges  are  sometimes  composed  of  an  indurated  organic  sand  or  a 
kind  of  fine  and  worn  shell-breccia.  Sponge  spicules  are  seldom  absent 
at  this  level  and  they  are  sometimes  present  in  great  profusion.  Some 
of  these  were  submitted  to  Dr.  J.  M.  Clarke,  who  identified  them  as 
root-tuft  fibres  of  some  hexactine  sponge,  presumably  a Hyalostelia. 
The  longest  are  usually  found  lying  horizontally  on  the  upper  surface 
of  the  ledges  and  measure  an  inch  and  a half  in  length.  Frequently 
cherty  layers  follow  the  seams  between  these  ledges  and  this  chert  is 
also  full  of  minute  spicules.  The  siliceous  character  of  the  ledges  is 
evidently  to  some  extent  due  to  infiltration.  In  the  calcareous  ledges 
were  noted : 

Sponge  spicules. 

Crinoid  stems. 

Productus,  sp. 

A bryozoan. 

Hustedia,  sp. 

A cyathophyllid  coral. 

Meekella,  sp. 

Thickness:  85  feet. 

The  Thin-bedded  Zone. — This  consists  of  a dark,  evenly  bedded  and 
compact  limestone  including  some  sandy  strata.  The  limestone  con- 
tains a considerable  amount  of  cherty  material,  which  weathers  out  in 
rusty  edges  of  plates  of  irregular  shapes  on  the  surface.  This  is 
especially  the  case  below.  Near  the  middle  the  chert  frequently 
assumes  the  form  of  spherical  shells  filled  with  material  like  the  body 
of  the  rock  and  solidly  imbedded  in  the  mass.  They  range  in  sizes 
from  three  to  six  inches  in  diameter.  Siliceous  material  is  also  found 
in  small  solid  spherical  nodules  about  half  an  inch  in  diameter.  Sug- 
gestions of  organic  structure — a Pachypora?  in  one  instance — occa- 
sionally appear  on  the  surface  of  these.  Frequently  all  the  fossils  in 
the  ledges  are  silicified  and  weather  out  in  perfect  form  with  the  rusty 
color  of  the  chert.  Fusulina  occurs  almost  everywhere  in  this  member, 
excepting  in  its  more  arenaceous  layers.  Under  a lens  the  body  of 
the  rock  is  usually  seen  to  be  made  up  of  a paste  of  minute  calcareous 
fragments  mostly  less  than  0.01  mm.  in  diameter.  In  this  there  are 
some  larger  organic  particles  and  fossil  remains,  mostly  such  as  joints 
of  crinoid  stems,  spines  of  brachiopods  and  bits  of  bryozoa.  Much  of 
this  rock  is  bedded  in  uniformly  thin  ledges  or  seams,  measuring  about 
four  inches.  Another  peculiarity  of  its  bedding  consists  in  occasional 
lenticular  thickenings  of  the  ledges,  which  increase  from  six  inches  to 
two  feet  and  again  run  out  to  the  same  dimensions  as  before,  all  in  a few 
feet  horizontally.  Where  least  affected  by  secondary  changes  a bitu- 
minous odor  is  sometimes  emitted  when  this  rock  is  crushed  under  the 
hammer. 


22 


Geology  of  the  Shafter 


The  following  fossils  were  noted  in  this  division: 

Fusulina  cylindrica  Fischer. 

Hyalostelia,  sp. 

Axophyllum  rude  White  and  St.  J. 

Campophyllum  torquium  Owen  [?]. 

Other  cyathophyllids. 

Crinoid  stems. 

Fistulipora,  sp. 

Other  bryozoa. 

Ambocoelia  planoconvexa  Shum. 

Chonetes,  two  species. 

Hustedia  mormoni  Marcon  [ ?]  a small  form. 

Meekella  striatocostata  Cox. 

Productus  costatus  Sow. 

P.  Mexicanus  Shum. 

P.  LongiSpinus  Sow. 

Pugnax,  sp. 

Seminula  argentea  Shep  [small]. 

Spirifer  cameratus  Morton. 

S.  hentuchensiSj  sp. 

Bellerophon,  sp. 

Aviculopecton,  sp. 

Pinna,  sp. 

Hungarites,  n.  sp. 

Horizontally  this  member  varies  considerably.  To  the  north  of 
Cibolo  creek  it  becomes  sandy  below,  and  to  the  south  it  contains  a 
considerable  lenticular  mass  of  thick-bedded,  brecciated,  grayish-white 
limestone.  Fusulina  is  sometimes  present  in  profusion,  while  at  other 
points  it  is  absent.  Elsewhere  the  rock  is  sometimes  argillaceous  and 
weathers  into  slaty  laminae.  The  base  is  not  always  clearly  limited,  and 
above  this  division  gradually  assumes  the  character  of  the  next  mem- 
ber. As  measured  in  the  hills  north  of  Cibolo  creek  these  beds  have  a 
thickness  of  470  feet. 

The  Yellow  Limestone. — The  highest  member  consists  of  a hard  yel- 
low and  dolomitic  limestone.  Its  lower  part  shows  bedding  planes, 
like  the  number  below,  but  higher  up  the  stratification  becomes  indis- 
tinct, and  single  ledges  appear  which  measure  twenty  or  even  forty  feet 
in  height.  The  rock  is  very  hard  and  contains  a considerable  amount 
of  siliceous  material,  which  probably  is  of  secondary  origin.  The 
silica  is  mostly  yellow,  like  the  rock,  and  is  more  or  less  evenly  distrib- 
uted in  the  mass  and  usually  not  forming  separate  nodules  or  layers. 
Indistinct  outlines  of  minute  organic  fragments  are  visible  under  a 
lens.  It  is  slightly  brecciated,  especially  above,  and  is  frequently  tra- 
versed by  minute  irregular  and  branching  fissures.  Now  and  then  there 
are  larger  irregular  fissure-like  pockets,  filled  with  white  quartz.  Fos- 
sils are  rather  scarce,  but  the  following  were  noted : 

1.  Fusulina  cylindrica  Fischer. 

2.  A fossil  with  somewhat  indistinct  texture  and  form,  like  that  of 
a Stromatopora. 

3.  Crinoid  stems. 

4.  Crinoid  heads.  [One  very  solid  block  was  seen  to  contain  a 
cluster  of  crinoid  heads,  crowded  together  in  a small  space.] 


Silver  Mine  District. 


23 


5.  Cleiothyris  roysii  L’Eveille  [?]. 

6.  Seminula  argentea  Shepard  [?]. 

This  member  is  exposed  at  only  one  place;  in  the  hills  north  of 
Cibolo  creek,  a half  mile  east  of  the  Cibolo  ranch.  Possibly  the  upper- 
most ledges  in  the  hill  at  the  silver  mine  may  belong  in  this  lower  part. 
Elsewhere  it  has  been  removed  by  erosion  before  the  Cretaceous  rocks 
were  laid  down.  The  latter  are  usually  found  resting  on  beds  which 
belong  farther  down.  Thickness:  650  feet. 

GEOGRAPHIC  CONDITIONS. 

It  is  clear  from  the  above  that  the  Chinati  series  comprises  the  greater 
part  of  one  single  sedimentary  cycle.  Downward  it  does  not  extend 
through  the  basal  deposits  of  this  cycle.  Nor  does  it  include  all  of  the 
limestones  which  belong  to  it  above,  for  the  uppermost  member  is  rep- 
resented merely  by  a small  outlier.  When  the  Cieneguita  conglomer- 
ates were  deposited,  the  shore  line  was  not  far  distant.  Undoubted 
plant  remains  occur,  and  there  are  ledges  of  beach-worif  pebbles  and 
sand.  The  granite  boulders  and  the  breccia  with  faceted  and  appar- 
ently extraneous  pebbles  suggest  that  icebergs  may  have  stranded  off 
this  shore,  and  increased  the  rate  of  sedimentation.  The  shore  line 
later  advanced  inland  and  these  deposits  were  then  buried  under  more 
uniformly  fine  material,  such  as  is  now  deposited  on  the  continental 
shelves  beyond  the  shore,  for  these  are  made  up  of  fine  sand  and 
silt,  usually  lacking  the  finest  mud  of  coast  lagoons.  This  silt  has  a 
sharply  marked  and  straight  lamination,  such  as  characterizes  very 
rapidly  accumulating  deposits.  As  the  depth  of  the  sea  kept  on  increas- 
ing, the  sand  no  longer  reached  this  far  out,  and  a calcareous  ooze  with 
fine  organic  fragments,  small  foraminiferal  casts,  slender  sponge 
spicules,  and  fusulinas  kept  on  accumulating.  This  stage  completed 
the  cycle.  Its  latest  sediments  were  carried  away  during  the  long 
period  of  elevation,  which  preceded  the  cretaceous  subsidence. 

In  general  the  whole  series  shows  a considerable  range  of  variability 
horizontally,  in  sections  at  different  places.  This  is  evidently  due  to 
diversity  of  attendant  geographical  conditions,  such  as  an  irregular 
shore  line,  a variable  depth  of  the  sea,  and  a diversity  of  currents  in 
the  depositing  medium. 

CORRELATIONS. 

The  fauna  of  the  Chinati  series  can  not  be  of  an  earlier  age  than  the 
later  part  of  the  Upper  Carboniferous.  It  has  a general  resemblance 
to  the  fauna  of  the  Missourian  in  the  Mississippi  basin,  which  is  the 
uppermost  and  latest  division  of  the  Carboniferous  age  and  whose  rocks 
are  continuous  with  the  sediments  of  the  Permian  on  the  plains.  The 
present  author  would  have  regarded  the  entire  series  as  belonging  to 
the  Upper  Carboniferous  except  for  the  evident  resemblance  in  its 
stratigraphic  succession  to  the  Guadaloupian  series,  which  lately  has 
been  pronounced  by  Girty  to  belong  to  the  Permian*  and  also  for  the 

*The  Upper  Permain  in  Western  Texas,  by  George  H.  Girty,  Am.  Jour,  of 
Sci.,  November,  1902,  p.  363. 


24 


Geology  of  the  Shafter 


occurrence  of  two  cephalopods  which  Smith  has  identified  as  forms  not 
previously  known  below  the  Permian.  The  Guadaloupian,  as  described 
by  Girty,  consist  of  some  1700  to  1800  feet  of  white  limestone,  under 
this  some  100  feet  of  dark  colored,  thinly  laminated  and  foliated  lime- 
stone, then  from  2000  to  2500  feet  of  yellow  sandstone,  and  farthest 
down  a black  limestone  only  partly  exposed  and  still  500  feet  thick. 
This  succession  suggests  an  equivalence  with  the  Chinati  series,  and  the 
occurrence  of  a Waagenoceras  and  of  a Hungarites  in  the  Cibolo  lime- 
stones emphasizes  the  suspicion  that  this  part  of  the  series  belongs  to 
the  Permian  age. 

Otherwise  the  paleontologic  evidence  is  decidedly  against  the  view 
that  the  two  series  are  parallels.  In  a general  way  the  two  faunas  are 
quite  unlike  each  other,  as  appears  in  the  following  comparison: 

CHARACTERISTICS  OF  THE  CHINATI  AND  THE  GUADALOUPE  FAUNAS 

COMPARED. 


Chinatian. 


Guadaloupian* 


1.  Fusulina  cylindrica  of  moderately 
large  size  and  not  of  very  slender  form. 

1.  A very  large  and  slender  form  of  fusu- 
lina. 

2.  Entire  sponges  apparently  absent,  but 
with  a great  abundance  of  separate  spicules 
of  siliceous  sponges  at  one  horizon. 

2.  Entire  [?]  sponges  abundant,  chiefly 
of  the  class  of  calcispongia. 

3.  Hydrocorallina  absent  or  very  rare. 
Cyathophyllids  common. 

3.  Hydrocorallina  not  rare,  other  corals 
rare. 

4.  Echinoderms  common.  Stems  of  cri- 
noids  most  frequent  of  all  fossils. 

4.  Echinoderms  not  observed. 

5.  Bryozoa  of  carboniferous  types. 

5.  Bryozoa  unlike  known  carboniferous 
forms. 

6.  Brachiopods  not  specially  numerous. 

6.  Brachiopods  very  common. 

7.  Pelecypods  represented  by  only  three 
or  four  fragments  in  the  collection  made. 

7.  Pelecypods  quite  numerous. 

8.  Gasteropods  fairly  common,  nearly  all 
forms  large. 

8.  Gasteropods  rare,  and  nearly  all  of 
minute  size. 

This  comparison  is  based  chiefly  upon  the  forms  found  in  the  upper 
limestones  in  each  of  the  two  sections.  Some  of  the  differences  enum- 
erated clearly  indicate  a difference  in  the  attendant  geographic  condi- 
tions. The  presence  in  the  Cibolo  limestones  of  siliceous  sponge  remains 
and  of  simple  forms  of  cyathopyllid  corals,  the  abundance  of  remains 
of  echinoderms,  and  the  paucity  of  pelecypods  all  indicate  the  condi- 
tion of  an  open  sea  some  distance  away  from  the  shore,  while  in  the 
Guadaloupian  fauna  the  presence  of  calcispongia,  and  of  hydrocoral- 
lina,  the  abundance  of  - brachiopods  and  pelecypods  and  absence  of  echino- 
derms all  testify  to  a greater  proximity  of  the  shore.  The  geographic 


^Characteristics  as  given  by  Girty.  Reference  as  above. 


Silver  Mine  District. 


25 


conditions  were  no  doubt  different,  whether  the  two  limestones  were 
made  at  the  same  time  or  not.  It  is  to  be  hoped  that  a more  thorough 
study  of  the  two  faunas  and  more  extensive  examination  of  the  terranes 
in  the  field  will  throw  more  light  on  the  question  of  their  relation- 
ship. With  our  present  knowledge  one  can  not  regard  it  as  impossible 
that  the  Capitan  limestone  of  Girty  may  turn  out  to  be  an  equivalent 
to  the  yellow  limestone  of  the  Cibolo  beds,  and  that  both  may  be  a 
part  of  the  Permian  cap  which  rests  on  top  of  the  Upper  Carbonifer- 
ous column  to  the  east  of  the  Cordilleran  region  in  the  southern  interior 
of  North  America. 

THE  CRETACEOUS  ROCKS. 

At  the  end  of  the  Paleozoic  era  the  bottom  of  the  sea  was  elevated 
and  its  sediments  were  submitted  to  erosion  during  the  Triassic  and 
Jurassic  ages.  Toward  the  end  of  the  latter  age  much  of  the  latest 
accumulations  of  the  Carboniferous  age  had  already  been  carried  away. 
Then  there  came  a new  subsidence  and  in  the  advancing  sea  the  Cre- 
taceous sediments  were  laid  down.  These  attained  a thickness  of  some 
three  or  four  thousand  feet.  Near  the  Chinati  mountains  the  later 
rocks  of  this  age  have  been  carried  away  by  erosion,  and  only  those 
now  remain,  which  represent  the  earlier  part  of  the  age,  the  Lower 
Cretaceous.  These  naturally  fall  into  five  successive  divisions,  or  for- 
mations, as  indicated  below. 

SYNOPSIS  OE  THE  SECTION  OF  LOWER  CRETACEOUS  ROCKS  NEAR  SHAFTER. 

Thickness 
in  feet. 


5.  Buda  limestone  [ ?]  70 

4.  Del  Rio  clay  [ ?] 80 

3.  Edwards  limestone 350 

2.  Shatter  beds 700 

1.  Presidio  beds 400 


THE  PRESIDIO  BEDS. 

West  of  the  new  shaft  of  the  Presidio  Mining  Company  there  is  a 
bluff  facing  the  Chinati  mountains,  which  rises  some  250  feet  from  a 
valley  running  nearly  north  and  south  and  in  the  south  of  which  the 
old  mining  village  is  located.  The  west  slope  of  this  valley  consists  of 
the  uncovered  upper  surfaces  of  the  ore-bearing  carboniferous  lime- 
stone. In  the  bottom  and  in  the  east  bluff  there  are  exposed  nearly 
400  feet  of  Cretaceous  sediments,  beginning  with  a clay  resting  directly 
on  the  Cibolo  limestone  and  ending  with  a solid  ledge  of  calcareous 
mortar  rock.  This  ledge  caps  several  other  hills  in  the  immediate 
vicinity  and  it  is  the  most  conspicuous  member  in  the  Cretaceous  series 
of  the  region,  the  Edwards  limestone  excepted.  The  details  of  the  sec- 
tion in  this  bluff  are  as  follows,  beginning  above.  (See  also  Fig.  5.) 


Fig.  5.— Section  of  the  Presidio  beds  in  the  bluff  west  of  the  shaft  of  the  Presidio  Mining  Company. 


Silver  Mine  District. 


27 


SECTION  IN  THE  BLUFF  DIRECTLY  WEST  OF  THE  NEW  SHAFT  OF  THE 
PRESIDIO  MINING  COMPANY. 

Thickness 
in  feet. 


27.  A strong  mortar  rock  of  somewhat  variable  composition.  The 
imbedded  sand  is  finest,  and  the  matrix  is  most  scanty, 
below.  The  middle  part  is  at  times  almost  free  from  sand 
and  contains  Orbitulina  texana  Roem.  The  uppermost 
ledges  contain  quite  coarse  sand  and  often  many  of  the 
grains  have  a well-developed  enveloping  oolitic  crust  of  cal- 
careous material.  Wholly  calcareous  oolitic  spherules  also 
occur.  Irregular  rusty  and  siliceous  streaks  weather  out  on 


the  surface  of  these  ledges,  and  they  contain  a large  pele- 

cypod  with  thick  valves,  resembling  a Cardium 30 

26.  Not  well  exposed,  but  apparently  largely  an  arenaceous  and 
argillaceous  soft  limestone,  with  a brecciated  ledge  below. 
This  number  contains  Orbitulina  texana  Roem.,  Trigonia 
emoryi  Con.,  and  two  sea-urchins  resembling  a Diadema  and 

a Toxaster 82 

25.  Sandy  shell  breccia  in  which  the  shell  fragments  are  of  small 

size  3 

24.  Dark  gray  limestone  with  imbedded  sand,  changing  into  a 


coarse  shell  breccia  above.  A large  gasteropod  like  Glaucoma 
branneri  Hill,  Pecten  n.  sp.,  Cucullcea  terminalis  Con.  (?), 


Lima,  sp.  (?),  Pholadomya  texana  Con.,  and  a large  Ostrea 

were  noted 29 

23.  Dark  gray  impure  sandy  limestone,  with  Orbitulina  texana.  . . 12 

22.  Sandstone  of  moderately  coarse  texture  and  light  gray  color.  . 15 

21.  Cray  limestone  containing  Orbitulina  texana 12 

20.  Gray  limestone  in  ledges  which  are  not  clearly  defined.  The 
lowermost  ledge  contains  thin  and  flat  valves  of  lamelli- 
branchs  of  small  size.  The  uppermost  ledges  have  a brecci- 
ated structure 28 

19.  Thin-bedded,  marly,  soft  material,  weathering  rapidly.  Not 

well  exposed 10 


18.  A grayish  white  limestone  helow,  mixed  with  some  fine  quartz 
sand.  Somewhat  softer  than  the  following  number.  Above, 
this  runs  into,  first,  an  equal  mixture  of  sand  and  calcareous 
material  and  then  into  a sandstone.  The  ledges,  or  laminae, 


measure  from  seven  to  eight  inches  in  thickness 29 

17.  Dark  gray  ledges  of  strong  sandstone,  in  which  a considerable 

part  of  the  sand  grains  are  calcareous.  7 


16.  Sandy  and  sometimes  cross-bedded  conglomerate,  containing 
water-worn  pebbles  of  limestone,  chert,  and  white  quartz, 
ranging  up  to  two  inches  in  diameter.  The  limestone  peb- 
bles are  the  largest  and  are  well  rounded.  The  chert  is 
mostly  angular  and  frequently  shows  fine  straight  spicules 
of  sponges  and  silicified  fusulinas.  The  white  quartz  is  more 


commonly  in  rounded  pebbles 14 

15.  A gray  limestone  of  fine  texture  with  a small  admixture  of  fine 
quartz  sand.  When  weathering,  it  takes  on  a concretionary 
appearance 4 


28  Geology  of  the  Sh  after 

14.  Not  well  exposed  but  apparently  for  the  most  part  consisting 
of  an  indurated  sandy  marl  below.  At  about  eight  feet  from 
the  base  a ledge  of  conglomerate  was  noted.  This  was  seen 
to  contain  pebbles  of  chert,  white  quartz,  quartzitic  sand- 
stone, and  limestone.  The  largest  pebble  seen  was  two  inches 
in  diameter.  A limestone  pebble  was  seen  to  contain  an 
imbedded  fusulina.  At  eighteen  feet  above  the  base  a brecci- 


ated  limestone  of  yellow  and  reddish  color  was  noted 26 

13.  Dark  limestone  of  a concretionary  structure 1 

12.  Fine-grained,  gray  sandstone 11 

11.  Gray,  sandy  and  calcareous  shell-breccia  of  a concretionary 

structure.  Dark  crystalline  calcite  noted 14 

10.  Light  gray  stony  marl  with  fine  yellow  sand  in  irregular 

laminae 6 

9.  Gray  laminated  shale,  interbedded  with  from  one-half  to  two 
inch  layers  of  fine  sandy  and  calcareous  material.  No  fos- 
sils noted 7 


8.  Gray  mortar  rock  consisting  of  calcareous  material  mixed  with 
fully  two-thirds  of  its  bulk  of  fine  sand.  This  is  interlamin- 
ated  with  layers  of  sandstone  and  shell-breccia,  above  and 
below.  Spheroidal  calcareous  concretions  and  some  fossils 


were  noted 14 

7.  Gray  marl 8 

6.  Gray,  almost  compact  limestone  with  a considerable  admixture 

of  imbedded  fine  sand  and  with  some  angular  gravel 2 

5.  Gray  marl 9 

4.  Hard  and  dark  limestone  with  dark  fragments  of  Ostrea 
valves  of  large  size  and  with  an  Exogyra  somewhat  like  E. 

texana 2 

3.  Gray  marl  changing  into  a sandy  shell-breccia  above 2 

2.  Gray  marl  with  round  concretions  of  a compact  calcareous 

material  and  measuring  from  two  to  six  inches  in  diameter.  . 6 

1.  Bluish  gray  clay 17 


When  the  new  shaft  of  the  Presidio  Mining  Company  was  sunk  at  a 
point  some  800  feet  to  the  east  of  the  crest  of  the  bluff  where  the  above 
section  is  exposed,  a careful  record  was  taken  of  the  strata  and  typical 
samples  of  the  rock  from  each  were  preserved.  These  samples  were 
carefully  examined  by  the  writer  in  the  office  of  the  company.  The  sec- 
tion of  the  shaft  as  given  in  the  company’s  record  and  verified  by  the 
samples  is  as  below,  leaving  out  the  Cibolo  limestone.  The  measure- 
ments given  are  those  from  the  record  of  the  shaft,  only  that  these  have 
been  corrected  for  the  dip,  which  it  about  15°,  so  as  to  represent  the 
actual  thickness  of  the  strata  on  a line  vertical  to  the  planes  of  the 
bedding. 

SECTION  OF  THE  NEW  SHAFT  OF  THE  PRESIDIO  MINING  COMPANY. 

Thickness 
in  feet. 

19.  A mortar  rock  in  which  the  grains  are  encrusted  with  concen- 
tric layers  of  calcareous  material.  Occasional  encrusted 
and  rounded  shell  fragments.  Calcareous  matrix  not  very 
copious.  Most  sand  grains  from  £ to  y1^  mm.  in  diameter.  34 


Silver  Mine  District.  29 

18.  A gray  sandstone  of  uniformly  fine  texture,  the  bulk  of  the 

grains  measuring  from  J to  ^ mm.  in  diameter 17 

17.  Yellowish  gray  sand  of  very  small  grains  from  to  ^ 

mm.  in  diameter,  and  with  some  calcareous  material 21 

16.  A dark  shell  breccia  of  more  or  less  worn  dark  shell  frag- 
ments of  all  sizes  held  in  a matrix  of  more  light  color. 

Ostrea  noted,  also  some  pyrite 72 

15.  An  almost  compact  limestone  containing  occasional  worn 

organic  fragments  and  some  quartz  sand 28 

14.  A gray  sandstone  with  a sparse  calcareous  cement 34 

13.  Limestone  of  dark  gray  and  compact  texture,  with  scattered 

angular  grains  of  quartz 11 

12.  Yellow  .limestone  composed  of  gray  rounded  organic  sand- 
grains,  most  from  TV  to  £ mm.  in  diameter.  These  are 
held  in  a sparse  calcareous  matrix  of  ochreous  color.  Crys- 
tals of  calcite  noted 14 

11.  Gray  mortar  rock,  with  quartz  grains  mostly  from  y1^-  to  \ 

mm.  in  diameter 2,8 

10.  A yellow  marl  with  occasional  angular  small  pebbles  of  chert.  14 

9.  A conglomerate  consisting  of  well  rounded  pebbles  of  lime- 
stone, chert,  and  vein-quartz,  mostly  from  one  to  two  centi- 
meters* in  diameter,  imbedded  in  a matrix  of  a fine  clastic 
paste.  A layer  of  crystalline  calcite  frequently  follows  the 

surface  of  the  larger  pebbles 40 

8.  Gray  mortar  rock  with  a copious  calcareous  matrix,  sand 

grains  mostly  from  y1^  to  ^ mm.  in  diameter 14 

7.  Gray  mortar  rock  with  a somewhat  sparse  calcareous  matrix. 

Grains  about  J mm.  in  diameter 14 

6.  A dark  organic  breccia  composed  of  shell  fragments  ranging 
up  to  three  centimeters  in  diameter,  and  containing  some 

sand  25 

5.  Brownish  gray  sandstone 2 

4.  Mortar  rock  of  dark  gray  color,  containing  about  one-third 

sand 42 

3.  Sandstone  of  dark  gray  color  with  a sparse  calcareous  matrix 
which  in  places  is  crystalline.  An  impression  of  a gastero- 

pod  noted 6 

2.  An  indurated  gray  marl  or  clayey  limestone,  showing  a few 

calcareous  and  organic  fragments.  Base  of  the  Cretaceous.  26 
1.  Yellow,  white,  and  bluish  limestones  belonging  to  the  Cibolo 
formation,  were  explored  by  the  shaft  below  this,  for  a dis- 
tance of  about 200 

General  Character. — Although  the  beds  included  in  the  above  sec- 
tions range  from  conglomerates  and  clay  to  limestones  and  sandstones, 
they  may  be  regarded  as  constituting  a lithological  unit  in  the  sense 
that  they  have  one  character  in  common  in  which  they  differ  from  the 
other  divisions  of  the  Lower  Cretaceous : they  are  all  mixtures  of 
mechanical  sediments,  such  as  gravel,  sand,  and  clay,  with  precipitated 
carbonate  of  lime  as  an  original  ingredient.  The  limestones  are  never 


*One  Centimeter=0.39  inch,  about. 


30 


Geology  of  the  Shafter 


free  from  sand  and  the  sandstones  are  hardly  ever  free  from  calcareous 
material.  This  is  not  the  character  of  the  sediments  which  overlie  the 
Presidio  beds.  The  oolitic  nature  of  the  uppermost  ledges  heralds  a 
coming  change,  when  the  shore  of  the  Cretaceous  sea  advanced  on  the 
existing  land  and  left  this  region  as  a more  open  sea,  where  the  deposits 
began  to  be  more  perfectly  sorted. 

Fossils. — Fossil  remains  are  not  rare,  especially  in  such  ledges  as  are 
in  part  made  up  of  organise  fragments.  But  they  are  frequently  broken 
up  in  weathering.  The  following  forms,  or  forms  resembling  them, 
were  noted: 

Minute  foraminifera. 

Orbitulina  texana  Roem. 

Astrocoenia  [?]  sp. 

Enallaster,  sp. 

Nerinea,  sp. 

Turritella  'planilateris  Conrad  [?]. 

Cylichna  recta  Gabb  [ ?] . 

Globiconcha  planata  Roem  [ ?] . 

Cerithium  [large  cast]  ? 

Trigonia  crenulata  Lam. 

Trigonia  tajji  Cragin. 

Cypricardia  texana  Roem. 

Ostrea  franklini  Coq. 

Ostrea  [a  large  form]. 

Pinna,  sp. 

Pholadomya  texana  Con  [ ?] . 

Pec  ten,  n.  sp. 

Cucullcea  terminates  Con  [?]. 

Correlation. — Lithologically  the  Presidio  beds  are  identical  with  R. 
T.  HilFs  Travis  Peak  beds  of  the  Grand  Prairie  region  and  the  two 
are  analogous  in  position.*  They  constitute  what  he  has  called  the 
ffbasement  sands”  of  the  Lower  Cretaceous.  Faunally  they  may  be  less 
closely  related,  but  it  is  significant  that  a section  of  the  Travis  Peak 
beds  which  is  seen  along  the  Colorado  river  nearly  four  hundred  miles 
to  the  east  should  present  a succession  of  sediments  quite  like  that 
of  the  beds  here.  In  both  places  there  is  a strong  mortar  rock 
above,  which  changes  downward  into  sandstone.  Under  this  are  some 
deposits  of  a finer  texture  and  below  these  again  lie  the  main  conglom- 
erates. In  both  places,  also,  these  basement  sands  extend  up  to  the 
lowermost  limestone  ledges  which  were  found  to  contain  Monopleura 
and  Requienia. 

The  outcrops  of  the  Presidio  beds  run  in  a belt  following  the  outer 
border  of  the  Carboniferous  rocks  east  and  south  of  the  Chinati  moun- 
tains. They  are  also  seen  bordering  the  Carboniferous  area  on  the 
southeast  in  the  Morita  creek  basin. 

THE  SHAFTER  BEDS. 

Upward  the  Presidio  beds  give  place  to  a long  succession  of  “alternat- 

*Black  and  Grand  Pariries,  Texas,  R.  T.  Hill,  U.  S.Geol.  Surv.,21st  An.  Rept., 
Vol.  VII,  p.  142  et  seq. 


Silver  Mine  District. 


31 


ing”  sediments,  measuring  some  700  feet  in  thickness.  These  consist 
of  ledges  of  limestone,  stony  marls,  clays,  and  fine  grained,  straight 
bedded  lenticular  masses  of  yellow  and  brown  sandstones,  all  recurring 
in  frequent  alternations.  In  the  entire  division  certain  fossils  are 
characteristic  of  certain  types  of  rocks,  and  reappear  sometimes  in 
slightly  changed  form,  at  successive  levels  in  the  same  kind  of  rock. 
Thus  Caprinas  are  seldom  absent  from  the  upper  surface  of  the  hardest 
limestone  ledges.  Bluish  and  clayey  marly  limestones  are  almost  cer- 
tain to  contain  Orbitulina  texana.  Thin  ledges  of  worn  shell  sand, 
which  sometimes  occur,  are  apt  to  have  spires  of  Turritellas.  White 
and  pure  limestone  ledges  are  usually  filled  with  minute  foraminifera. 
The  sandstones  are  not  fossiliferous. 

These  sediments  I shall  call  the  Shatter  beds.  They  are  exposed  in 
all  the  hills  immediately  around  Shatter  and  extend  to  the  south,  south- 
east, and  southwest  a distance  of  several  miles  from  the  town  of  this 
name,  until  they  disappear  beneath  the  Edwards  limestone.  A nearly 
continuous  section  of  these  beds  was  worked  out  from  exposures  seen 
at  several  different  points.  (See  Fig.  5.)  The  lowermost,  which  rest 
on  the  Presidio  beds,  come  into  view  in  an  arroyo  about  two-thirds  of  a 
mile  to  the  east-south-east  of  the  new  shaft  of  the  Presidio  Mining  Com- 
pany. These  may  be  described  as  follows: 

EXPOSURE  i. 

Thickness 
in  feet. 

10.  Hard  gray  limestone,  in  part  composed  of  small  organic  frag- 


ments and  containing  Caprina  crassifibra  Roem 6 

9.  Soft  gray  calcareous  rock  with  Orbitulina  texana  Roem.  in 

great  abundance.  Porocystis  pruniformis  Cragin  also  noted.  7 
8.  Yellowish  sandstone 4 


7.  Soft  marly  gray  rock  containing  Orbitulina  in  its  upper  part.  39 
6.  Somewhat  harder  ledges,  consisting  of  organic  fragments 
imbedded  in  a structureless  mass.  Small  foraminiferal 
shells  and  occasional  Caprinas’  are  imbedded  in  the  mass ....  6 

5.  Soft  marly  materials,  not  well  exposed 15 

4.  A calcareous,  gray  rock,  consisting  in  part  of  organic  frag- 
mental material,  and  occasionally  containing  Caprina  cras- 
sifibra Roem.  and  a large  gasteropod.  The  upper  surface  of 
the  ledge  is  traversed  by  vertical  rusty  streaks  about  one- 
third  of  an  inch  in  diameter  and  these  weather  out  into  hol- 


low pits 7 

3.  Material  like  the  above,  but  somewhat  softer  and  more  plainly 

laminated 13 

2.  Gray  limestone,  containing  shells  of  small  foraminifera  and  a 
Monopleura.  The  latter  is  silicified  and  usually  stands  in  a 

vertical  position,  weathering  out  with  a rusty  color 3 

1.  Gray  soft  limestone,  not  well  exposed 2 


In  the  west  bank  of  Cibolo  creek  below  Shafter  a continuous  section 
was  made  out,  of  about  260  feet.  This  includes  nearly  all  of  the  sec- 
tion already  given.  It  runs  as  below. 


32 


Geology  of  the  Shafter 


23. 

22. 

21. 

20. 

19. 

18. 

17. 

16. 

15. 

14. 

13. 

12. 


11. 


10. 


9. 

8. 


7. 


6. 


5. 


EXPOSURE  II. 


Thickness 
in  feet. 


Dark  gray  limestone,  its  upper  surface  with  a mottled  appear- 
ance   2 

Gray  marl 3 

Gray  limestone,  compact  below  and  with  minute  foraminiferal 

casts  above 2 

Gray  marl 2 

Gray  limestone,  like  number  19  above 3 

Gray,  somewhat  indurated  marl 2 

Gray  limestone,  like  numbers  19  and  17  above 2 

Gray  stony  marl 2 

Gray  limestone  weathering  into  rounded  pieces 3 

Gray  stony  marl 1 

Gray  solid  limestone  in  three  somewhat  indistinct  equal  ledges. 

Occasionally  Monopleuras  and  Caprinas  are  seen 8 

Limestone  like  the  previous  number.  Near  the  base  there  is  a 
seam  in  which  Monopleura  marcida  White,  standing  verti- 
cally, is  abundant.  A large  and  thick  lamellibrauch  was  also 

noted 7 

Limestone  like  the  above,  but  full  of  Orbitulina  texana  Roem., 
and  with  a slightly  softer  stratum  near  the  middle.  Fos- 
sils noted':  a gasteropod,  a sea  urchin,  a small  Gryphaea,  and 
a Pecten  occidentalis  Con 12 


[The  numbers  which  follow,  appear  some  distance  further 
down  in  the  same  side  of  the  creek.] 

Gray  marly  limestone  with  Orbitulina  texana  Roem.  in  abund- 
ance. Other  fossils:  Ostrea,  sp.,  Porocystis  pruniformis 


Cragin,  Toxaster,  sp.,  and  some  gasteropods.  The  number 
contains  one  thin  ledge  of  more  compact  texture  at  about 

seventeen  feet  below  the  top 65 

Laminated  gray  compact  limestone,  shaly  above 3 

Massive  gray  limestone,  weathering  yellow,  and  largely  made 
up  of  organic  fragments,  occasionally  containing  Orbitulina. 
Fossils:  a small  Ostrea  and  Alectryonia  diluviana  Lam.  ( ?) . 7 

Gray  stony  marl  with  Orbitulina  texana  Roem 15 

Sandstone,  rather  coarse  above  and  changing  below  into  silt  and 


clay,  very  sharply  marked  off  from  the  limestone  above.  In 
an  uplifted  block,  some  distance  farther  down  where  there  is 
a high  bluff  on  the  west  side  of  a bend  of  the  creek,  this  sand- 
stone is  thirteen  feet  thick.  It  breaks  into  sharply  angular 
blocks  and  the  upper  two  feet  are  finely  laminated.  It  is 

also  in  places  obliquely  bedded 5 

[The  numbers  which  follow  are  seen  in  this  uplifted  block.] 
Gray  limestone  with  Orbitulina,  hardest  below,  and  containing 
Nerinea  texana  Roem.,  Nerinea,  n.  sp.,  Pecten  occidentalis 
Con.  and  some  imperfect  specimens  of  forms  resembling 


Avicula,  sp.,  Astarte,  sp.,  and  Idonearca,  sp.[?] 9 

Fine-grained  yellowish  sandstone  breaking  vertically  and  with 
a very  sharply  marked  base 20 


Silver  Mine  District. 


33 


4.  Gray  shale,  yellow  above,  bluish  green  below,  with  a calcareous 
seam  14  inches  from  top,  which  has  its  upper  surface  liter- 
ally covered  with  Orbitulina.  The  entire  shale  is  finely 


laminated 7 

3.  Gray  soft  marl  with  Orbitulina  texana  Roem 30 


2.  Gray  clayey  marl  alternating  with  more  indurated  bands  of 
compact  calcareous  material.  Near  the  middle  these  alternat- 
. tions  are  thin  and  very  regular,  each  from  two  to  three 
inches  in  thickness.  Orbitulina  appears  to  be  absent  from 
this  number,  but  it  has  white  casts  of  minute  foraminifera. 

Above  and  below  the  material  is  somewhat  stony 34 

1.  Several  alternating  ledges  of  gray  limestone  and  stony  marl, 
both  of  which  are  made  up  of  minute  organic  fragments, 
among  which  some  minute  tests  of  foraminifera  may  be  seen.  17 
Numbers  6 to  10  in  exposure  I are  the  equivalents  of  numbers  1 to 
5 in  exposure  II.  It  is  possible  that  some  ledges  have  been  left  out  in 
-exposure  I,  for  the  ground  was  somewhat  broken  up,  but  the  discrep- 
ancy can  at  most  involve  only  a few  feet.  Evidently  the  lowest  num- 
ber in  exposure  II  comes  down  to  within  a close  distance  of  the  upper- 
most ledges  of  the  Presidio  beds. 

In  the  south  slope  of  the  second  arroyo  which  opens  into  Cibolo  creek 
from  the  west,  south  of  Shafter,  and  to  the  south  of  the  mill  belonging 
to  the  Presidio  Mining  Company,  I found  a section  which  continues  the 
previous  exposure  upward.  This  is  as  below: 


32. 

31. 

30. 


■29. 

28. 

27. 

26. 

25. 

24. 

23. 

22. 

21. 


20. 


EXPOSURE  III. 


Thickness 
in  feet. 


Hard  gray  limestone  with  Caprina  crassiftbra  Roem 4 

Gray  marly  rock  with  Orbitulina  texana  Roem 20 

Massive  gray  limestone  in  one  single  ledge  which  breaks  into 
large  blocks  with  rounded  edges.  Its  upper  surface  shows  a 

small  Gryphea,  and  Pecten  occidentalis  Con 4 

Stony  gray  marl  11 

Cross  bedded  yellow  sandstone  of  uniform  texture.  This  ledge 

sometimes  runs  out  suddenly * 12 

Stony  gray  marl 9 

Gray  marly  limestone  with  yellow  blotches  above 3 

Gray  stony  marl 4 

Gray  limestone  with  yellow  impregnated  patches  above 2 

Gray  stony  marl 5 

Gray  marly  limestone  with  some  small  oysters  or  gryphaeas  on 
its  upper  surface 3 


Stony  gray  marl,  not  well  exposed,  with  some  very  large 
moulds  of  gasteropods  and  some  large  lamellibranchs,  and 
Pecten  occidentalis  Roem.  Two  thin  indurated  ledges  were 
noted  at  33  and  21  feet  above  its  base.  At  about  3 feet  above 
its  base  a ledge  of  sandstone  is  seen  on  the  opposite  side  of 

the  arroyo 35 

A gray  shell  breccia  of  fine  texture  weathering  into  square 
blocks.  The  upper  surface  of  this  number  is  very  straight. 
Slender  and  straight  spires  of  gasteropods  are  frequent.  In 


34  Geology  of  the  Shafter 

places  the  upper  surface  is  marked  by  straight  and  ferrugin- 
ous crack-like  joints.  A quarter  of  a mile  further  down  the 
creek  this  ledge  is  replaced  by  about  10  feet  of  brown  sand- 


stone   1 

19.  Stony  gray  marl 3 


18.  Strong  ledge  of  fine-grained  limestone,  in  part  composed  of 
organic  fragments  and  also  containing  tests  of  small  forum  - 
inifera  above.  The  rock  weathers  brownish  yellow  and 
breaks  into  square  blocks.  From  its  upper  surface  extends 
into  it  a number  of  irregularly  cylindric  and  sinuous  tra- 


versions  of  oxidized  yellow  rock 3 

17.  Gray  limestone  containing  Monopleura  marcida  White  in  pro- 
fusion and  also  a Caprina 1 

16.  Gray  stony  marl,  almost  a shell-breccia  near  the  middle  and  in 

the  upper  part 11 


15.  Fine  compact  gray  limestone  with  a very  straight  obscure  lam- 
ination, weathering  into  straight  rectangular  blocks 

14.  Shaly  gray  marl 

13.  Gray  limestone  of  fine  texture,  compact  below,  and  with  a 
profusion  of  shells  of  minute  foraminifera  above.  When 
weathered  the  upper  surface  exhibits  a structure  like  that  of 

a conglomerate  or  breccia 

12.  An  interrupted  thin  seam  of  a compact  fine-textured  lime- 
stone   

11.  Marly  gray  limestone  with  imbedded  fragments  of  shells 

10.  Limestone  of  fine  texture  and  gray  color,  with  imbedded  tests 

of  minute  foraminifera 

9.  Compact  gray  limestone 

8.  Gray  marly  limestone 

7.  Gray  limestone  filled  with  casts  of  small  foraminifera.  Sinu- 
ous yellow  tra versions  extend  downward  from  the  upper  sur- 
face   

6.  A thin  seam  of  gray  marl 

5.  A compact  gray  limestone  containing  scattered  fragments  of 

shells 

4.  Gray  marly  limestone 

3.  Gray  limestone  in  two  ledges,  the  uppermost  thicker  and  con- 
taining a profusion  of  small  foraminifera,  the  lower  more 

compact  and  with  scattered  dark  organic  fragments 

2.  Gray  limestone : 

1.  Gray  limestone  consisting  of  a breccia  of  organic  fragments  of 
small  size.  Casts  of  minute  foraminifera  and  of  Orbitulina 

texana  Roem.  in  considerable  numbers 2 

Numbers  16  to  23  in  exposure  II  are  identical  with  numbers  1 to  13 
in  exposure  III.  These  numbers  contain  a thrice  repeated  cycle  of 
slightly  different  calcareous  ledges  which  are  clearly  shown  in  several 
places,  always  the  same,  and  everywhere  easily  identified..  The  upper 
ten  or  twelve  numbers  in  the  last  named  exposure  are  the  beds  on 
which  the  town  of  Shafter  is  built  and  they  are  also  exposed  in  the 
north  and  east  bank  of  Cibolo  creek,  on  the  opposite  side  from  the  town. 
In  the  hills  to  the  north  and  east  they  are  overlaid  by  a series  of  sedi- 
ments of  very  much  the  same  character  as  those  already  described, 


to  to  tOrc|M  itOtoli-1  CO  CO^Im  CO  to  to 


Silver  Mine  District. 


35 


including  marls,  shales,  limestones,  and  sandstones  in  continuous  alter- 
nations. Below  is  given  a section,  which  begins  in  the  bed  of  the  creek,, 
and  therefore  includes  the  uppermost  of  the  previous  exposure,  but 
which  continues  some  250  feet  higher  up. 


33. 

32. 


31. 

30. 

29. 

28. 

27. 


26. 

25. 

24. 

23. 

22. 

21. 


20. 

19. 


18. 

17. 

16. 

15. 


14. 


EXPOSURE  IV. 


Thickness, 
in  feet. 


Compact  grayish  white  limestone 

Marly  gray  limestone  with  Modiola  pedernalis  Roem.,  Neri- 
nea,  sp.,  Rostellaria  collina  Conrad  ( ?),  Natica  texana  Con- 
rad, Exogyra  texana  Roem.,  Engonoceras,  near  complication 

Hyatt,  and  Toxaster,  sp 

Dark  yellow  limestone  consisting  of  calcareous  organic  frag- 
ments in  a compact  paste 

Marly  limestone 

Compact  light  gray  limestone  with  scattered  imbedded  grains 

of  dark  color 

Marly  limestone  with  a more  indurated  ledge  at  about  four 

feet  from  the  base  

Compact  gray  limestone  filled  with  casts  of  minute  foramini- 
fera  in  the  lower  part.  The  upper  surface  of  this  ledge 
shows  some  irregular  oxidized  yellow  patches  which  extend 

into  the  ledge  as  irregular  finger-like  traversions 

Gray  marl,  somewhat  stony  above 

Sandstone,  rather  fine-grained^  'soft  below,  indurated  and 

weathering  into  straight  brown  blocks  above 

Gray  marl 

Straightly  laminated  gray  sandstone,  weathering  yellow  and 

brown 

Gray  marl 

Indurated  sandstone  of  uniform  texture,  cross-bedded,  with 

laminae  very  straight 

Gray  marl 

Gray  limestone  in  thin  layers  alternating  with  softer  nodular 
calcareous  material.  In  the  upper  part  of  this  number  there 
are  imbedded  some  dark  small  nodules,  which  give  the  rock 
a blotchy  appearance.  A ground  specimen  shows  some 
imbedded  tests  of  small  foraminifera.  Some  gasteropods 

present 

Gray  marl 

Stony  marl  with  a somewhat  concretionary  structure.  A 

very  large  mold  of  a pelecypod  noted 

Gray  shale 

Marly  clay  below^  with  thin  seams  of  somewhat  brecciated  gray 
limestone  above.  A small  Ostrea  and  a small  Gryphaea  were 
noted  and  also  casts  of  one  large  gasteropod  and  one  large 

pelecypod 

Gray,  and  yellowish  white  sandstone,  of  uniformly  fine  texture, 
some  with  beautiful  ripples  from  two  to  five  inches  across. 
It  usually  weathers  into  sharply  rectangular  blocks  which 
sometimes  split  up  into  thin  laminae 


4 


28- 


2 

1 

2 


13 


4 

18 

6 

12 

H 

16 

2 

15 


6 

12 

16 

6 


5 


14 


36 


Geology  of  the  Shafter 


13.  Marly  gray  limestone  with  casts  of  pelecypods,  changing  into 

clay  above 14 

12.  Gray  limestone  with  many  minute  tests  of  foraminifera,  pele- 
cypods  and  a Serpula.  Orbitulina  texana  Eoem.  also 

noted 3 

11.  Clay  with  a ripple-marked  sandy  seam  at  about  the  middle. 
Below  this  seam  the  clay  is  dark,  while  above,  it  is  lighter, 

arenaceous,  and  calcareous  and  has  some  fossils 14 

10.  Gray  marly  limestone,  soft  above,  filled  with  Orbitulina  texana 
Boem.,  also  sea-urchins  and  a Pecten,  a Gryphsea,  and 

Ostrea,  and  a small  gasteropod 20 

9.  A compact  gray  limestone  with  Caprina  crassifibra  Roem 3 


8.  Marly  limestone,  bluish  gray,  weathering  into  small  chips  of 
irregular  shape,  with  some  more  indurated  ledges,  especially 
above.  The  whole  number  is  almost  filled  with  Orbitulina. 
Other  fossils:  Sea  urchins,  Pecten  occidentalis  Con.,  Phola- 


domya,  sp 30 

7.  Massive  gray  limestone  weathering  into  large  irregular  blocks 
which  turn  yellow.  On  the  upper  surface  of  the  ledge  were 
seen  a Cardium,  Pecten  occidentalis  Con.,  and  a small  Gry- 
phaea 4 


6.  A gray  marl  with  casts  of  various  pelecypods  and  gasteropods.  16 


5.  Limestone  of  gray  color,  with  some  very  large  gasteropods  and 

Trigonia,  sp 2 

4.  Nodular  semi-indurated  marl  with  casts  of  several  pelecypods.  3 

3.  Gray  limestone 2 

2.  Shaly  marl 2 

1.  Gray  limestone  containing  frequent  imbedded  thin  and  small 
shells  mostly  in  fragments  and  also  casts  of  minute  fora.- 
minifera 5 


Up  to  the  highest  ledge  in  this  exposure  the  section  is  continuous  from 
the  top  of  the  Presidio  beds  and  it  measures  about  625  feet.  There  is 
evidently  some  more  of  the  same  kind  of  material.  I estimate  that 
there  is  at  least  twenty-five  feet  more.  In  the  last  exposure  there  is 
about  two  hundred  feet  of  beds  above  the  highest  orbitulina  marl.  In 
a section  which  is  seen  under  the  Edwards  limestone  at  the  junction  of 
Morita  creek  with  the  Ciholo,  about  six  miles  south  of  Shafter,  the 
highest  stratum  containing  Orbitulina  lay  about  230  feet  below  the 
base  of  the  Edwards  limestone.  The  character  of  these  beds  is  akin  to 
that  of  the  uppermost  in  the  last  exposure  [Exposure  4],  as  may  be  seen 
from  the  description  which  follows. 


5. 

4. 

3. 

2. 


EXPOSURE  V. 


Thickness 
in  feet. 


Massive  limestone  [Edwards] 200-300 

Impure  marly  limestone  of  light  gray  color  with  Exoqyra 

texana  Roem.  in  abundance 75 

Like  the  above,  but  with  some  more  indurated  ledges, 

Exogyra  less  frequent 60 

Reddish  brown  fine  sandstone,  of  very  uniform  texture, 

occasionally  ripple-bedded 

Shaly  gray  soft  limestone 


1. 


15 

80 


Silver  Mine  District. 


37 


Below  No.  1 there  is  a marly  gray  rock  filed  with  Orbitulina  texana 
Roem.  It  seems  that  most  of  this  exposure  is  included  in  the  upper 
part  of  exposure  IV,  and  if  we  place  the  uppermost  orbitulina  marl  at 
the  same  level  in  both,  it  will  be  seen  that  the  Edwards  limestone  should 
begin  about  twenty-five  feet  above  No.  33  in  exposure  IV.  This  would 
then  place  the  base  of  the  next  division,  the  Edwards  limestone,  at  an 
elevation  of  680  feet  above  the  top  of  the  Presidio  beds  and  make  this 
figure  the  thickness  of  the  Shatter  beds.  The  approximate  correct- 
ness of  this  figure  was  corroborated  by  another  rough  measurement  of 
the  same  beds  at  a point  three  miles  southwest  of  Shatter,  where  there 
is  a continuous  but  partly  concealed  exposure,  extending  from  north  to 
south  and  made  out  to  include  about  700  feet.  The  latter  figure  I 
believe  is  a conservative  estimate  of  the  thickness  of  the  Shatter  beds. 

Fossils  of  the  Shafter  Beds. — There  are  a number  of  fossils  in  these 
beds,  some  of  which  have  evidently  not  yet  been  described.  The  most 
conspicuous  ones  are  some  casts  of  large  pelecypods  and  gasteropods. 
I append  the  following  list  of  such  known  forms  as  seem  to  resemble 
those  occurring  in  these  beds. 

1.  Foraminifera  of  minute  size.  These  are  present  in  most  of  the 
indurated  ledges,  very  often  quite  abundant. 

2.  Orbitulina  texana  Roem.  Present  in  most  of  the  soft  dark  gray 
maids,  excepting  the  upper  part  of  the  section. 

3.  Porocystis  pruniformis  Cragin.  Present  at  two  horizons:  at 
about  170  feet  from  the  base  and  near  the  top  of  the  section. 

4.  Diadema  texanum  Roem. 

5.  Holectypus  Planatus  Roem. 

6.  Toxaster  texanus  Roem. 

7.  Serpula,  sp. 

8.  Lunatia  pedernalis  Roem.  Present  mostly  in  the  gray  marls. 

9.  Natica  texana  Con. 

10.  Nerinea  texana  Roem. 

11.  Nerinea,  n.  sp. 

12.  Rostellaria  collina  Con  [?]. 

13.  Rostellaria  mtexana  Con  [?]. 

14.  Rostellaria,  sp. 

15.  Turritella,  sp.  Occurs  most  in  ledges  consisting  of  worn  organic 
sand. 

16.  Avicula  (?)  [a  large  species]. 

17.  Caprina  crassifibra  Roem. 

18.  Caprina  occidentalis  Con.  This  and  the  previous  number  occur 
imbedded  in  the  upper  face  of  nearly  all  of  the  hard  ledges  of  any  con- 
siderable thickness. 

19.  Cardita  subtetrica  Con  [?]. 

20.  Cardium  mediale  Con. 

21.  Cucullcea  terminalis  Con  [?]. 

22.  Exogyra  texana  Roem.  Common,  especially  in  the  upper  200 
feet. 

23.  Lima  wacoensis  Roem  [?]. 

24.  Modiola  pedernalis  Roem. 

25.  Monopleura  marcida  White.  Occurring  near  the  base  in  one 
ledge  and  also  in  two  ledges  about  250  feet  above  the  base. 

26.  Ostrea  subovata  Shum  [?]. 


38 


Geology  of  the  Shafter 


27.  Ostrea,  sp.  ind. 

28.  Ostrea  [like  diluviana]. 

29.  Pecten  occidentalis  Con.  Is  common  in  the  hard  ledges. 

30.  Pecten  mexicanus  Roem. 

31.  Pholadomya,  sp. 

32.  Requienia,  sp. 

33.  Trigonia  emoryi  Con. 

34.  Trigonia  texana  Con. 

35.  Pngonoceras,  near  complicatum  Hyatt.  This  occurs  in  the 
upper  200  feet  of  the  section. 

36.  Small  hemispherical  tooth  of  a vertebrate.  Occurs  with  a 
Herinea. 

Correlations. — It  is  quite'  evident  that  the  lower  four  or  five  hundred 
feet  of  the  Shafter  beds  are  to  be  regarded  as  the  western  equivalent 
of  the  Glen  Rose,  as  delimited  by  R.  T.  Hill  in  his  Geology  of  the  Black 
and  Grand  Prairies,  Texas.*  Here  as  in  the  eastern  part  of  the  State, 
Requienia  and  Caprina  suddenly  appear  in  the  more  indurated  ledges 
overlying  the  basal  sands.  Lithologically  the  Shafter  beds  and  the  Pre- 
sidio beds  are  quite  dissimilar,  for  while  the  latter  consist  of  mixtures  of 
calcareous  and  arenaceous  materials,  the  limestones  and  the  marls  of 
the  former  seldom  if  ever  contain  any  admixture  of  sand,  and  when 
sandy  beds  occur  they  are  free  from  marly  material,  and  the  sand  is 
thoroughly  sorted  and  washed,  and  frequently  ripple-bedded. 

It  may  be  that  some  of  the  sandy  beds  in  the  upper  half  of  the  sec- 
tion are  to  be  regarded  as  homotaxial  equivalents  of  the  Paluxy  sands. 
But  the  sandy  beds  here  are  individually  not  of  any  great  horizontal 
extent  and  they  were  evidently  formed  as  submerged  bars,  probably  in 
deep  water.  They  run  out  abruptly  and  are  then  in  many  cases 
replaced  by  thin  ledges  of  limestone,  which  are  made  up  of  worn  organic 
sand.  Though  these  sandy  lenticular  masses  have  their  greatest 
development  at  from  100  to  300  feet  below  the  top  of  the  formation 
the  lowest  ones  occur  less  than  100  feet  from  its  base,  and  must  neces- 
sarily be  classified  with  that  part  of  the  formation  which  is  equivalent 
to  the  Glen  Rose.  The  texture  and  other  lithological  characters  of  all 
of  these  lenticular  masses  of  sandstone  is  so  uniform  throughout  that 
there  can  be  no  doubt  about  the  continuance  of  identical  physical  con- 
ditions during  the  time  of  their  making.  This  is  one  of  the  reasons 
why  the  present  author  regards  the  Shafter  beds  as  a unity. 

It  is  nevertheless  confidently  believed  that  th£  upper  part  of  these 
beds  will  be  found  to  belong  to  the  horizon  of  the  Walnut  clays.  It  lies 
under  the  base  of  the  massive  ledges  of  the  Edwards  limestone,  the 
lower  part  of  which  may  correspond  to  the  Comanche  Peak  limestone. 
The  uppermost  200  feet  consist  of  marls  that  are  more  white  and  chalky 
than  the  middle  and  lower  part  of  the  Shafter  beds,  and  they  contain 
an  Engonoceras.  Two  specimens  of  this  fossil  were  found  on  the  hills 
northeast  of  Shafter.  But  decisive  evidence  of  the  correctness  of  this 
conclusion  can  not  now  be  given. 

Geographical  Conditions. — The  Shafter  beds  were  deposited  in  a more 
open  and  perhaps  also  a deeper  sea  than  the  Presidio  formation.  The 

*21  An.  Rep.  U.  S.  Geol.  Survey.  Part  VII. 


Silver  Mine  District. 


39 


bottom  of  this  sea  seems  to  have  been  subject  to  frequent  oscillations, 
but  its  currents  were  probably  more  steady  and  sediments  less  copious, 
and  thus  they  were  more  perfectly  sorted.  The  lesser  inequalities  of 
the  bottom,  which  already  had  a covering  of  some  400  feet  of  sediments, 
naturally  aided  in  making  the  sorting  currents  steady  and  in  giving 
them  a greater  sway. 

THE.  EDWARDS  LIMESTONE. 

The  Shatter  beds  are  overlaid  by  a massive  white  limestone  in  heavy 
ledges.  This  forms  the  walls  of  a beautiful  canon  in  Cibolo  creek  six 
miles  south  of  Shatter.  Its  walls  rise  to  a height  of  400  feet  above  the 
bed  of  the  creek,  and  consist  for  the  most  part  of  this  limestone.  The 
same  formation  rises  to  a conspicuous  ridge  extending  to  two  miles 
from  west  to  east,  three  miles  south-southwest  of  Shatter.  From  the 
east  end  of  this  ridge  its  outcrop  is  shown  by  a more  subdued  divide, 
which  first  trends  to  the  southeast  and  then  to  the  east,  crossing  the 
Cibolo  at  the  canon  already  referred  to. 

A safe  estimate  of  the  thickness  of  this  limestone  makes  it  not  less 
than  350  feet.  It  probably  includes  the  Comanche  Peak  limestone 
below  and  the  Georgetown  limestone  above,  but  the  main  and  middle 
part  is  clearly  and  without  the  least  doubt  the  western  continuation  of 
the  Edwards  limestone.  My  time  in  the  field  did  not  allow  of  a 
detailed  study  of  its  ledges,  but  some  diagnostic  features  were  noted 
which  were  unmistakably  those  of  the  Edwards  limestone.  Seams  of 
flat  cakes  of  gray  flint  occurred  some  100  feet  above  the  base.  There 
were  also  some  white  ledges  of  a crystalline  texture  such  as  occur  in  the 
Edwards  limestone  elsewhere.  One  specimen  of  the  white  limestone, 
when  ground  and  polished,  was  seen  to  hold  in  a transluscent  calcar- 
eous matrix  a profusion  of  white  grains,  some  of  which  were  casts  of 
minute  foraminifera,  some  worn  organic  fragments,  some  oolitic  accre- 
tionary spherules  and  some  organic  fragments  with  a white  oolitic  coat- 
ing. Here  and  there  in  this  mass  were  crystals  of  calcite  with  irregular 
external  shape  but  perfect  internal  cleavage,  measuring  from  five  to  ten 
millimeters  in  diameter.  Two  other  specimens  of  a dark  gray  color 
had  a much  more  copious  and  partly  finely  fragmental  matrix,  in  which 
were  imbedded  thin  small  fragments  of  flat  shells  and  some  entire  casts 
of  minute  foraminifera. 

THE  DEL  RIO  CLAY  ( ?)  . 

The  Edwards  limestone*  dips  to  the  south  along  the  Cibolo  creek 
canon  and  in  about  a mile  and  a half  south  of  the  north  end  of  the 
canon,  a clayey  member  covers  its  upper  surface.  This  clayey  member 
is  some  eighty  feet  thick  and  it  contains  occasional  thin  ledges  of  lime- 
stone, some  of  which  are  crowded  with  small  spires  of  long  and  straight 
gasteropods.  During  a fifteen  minutes  search  I failed  to  find  Exogyra 
arietina  Roem.,  which  is  usually  always  present  in  the  Del  Rio  clay. 
Whether  this  clayey  member  is  to  be  referred  to  the  Del  Rio  or  not. 
must  hence  for  the  present  be  left  an  open  question.  It  certainly  is 

*The  term  must  here  be  understood  in  a loose  way.  designating  a limestone 
which  probably  includes  the  Comanche  below  and  the  Georgetown  above. 


40 


Geology  of  the  Shafter 


more  calcareous  than  the  latter  is  near  Terlingua  in  Brewster  county, 
eighty  miles  to  the  east. 

THE  BUDA  LIMESTONE  ( ?) . 

Above  this  clay  there  is  again  another  limestone  about  seventy  feet  in 
thickness.  This  consists  of  strong  ledges  of  a gray  color.  A single 
specimen  of  the  rock  was  examined  closely  and  was  seen  to  consist  of 
a compact  or  finely  fragmental  copious  matrix  with  obscure  organic 
fragments.  No  fossils  were  looked  for,  and  the  indicated  stratigraphic 
equivalence  of  the  rocks  is  merely  an  inference  from  its  position.  If 
any  of  the  upper  cretaceous  series  come  into  view  within  this  area  they 
will  be  found  along  Cibolo  creek  south  of  this  point.  Looking  at  the 
country  from  a distance  it  appeared  likely  that  all  the  rocks  above  the 
last  limestone  are  covered  by  the  land  drift. 

THE  LAND  DRIFT. 

Much  of  the  uplands  is  covered  by  a product  of  land  destruction, 
which  may  be  called  a land  drift.  It  consists  of  a heterogenous  mix- 
ture of  angular  and  rounded  boulders,  pebbles,  sand,  and  clay,  which 
evidently  has  been  brought  together  and  deposited  where  it  now  lies  by 
the  same  agents  of  disintegration,  erosion,  and  transportation,  that  are 
now  at  work  in  this  region.  When  we  find  this  drift,  as  we  sometimes 
do,  from  two  to  three  hundred  feet  deep,  it  is  difficult  to  conceive  of 
how  the  present  drainage  can  have  laid  down  such  great  quantities  of 
material,  but  when  we  begin  to  estimate  the  quantity  of  the  rock 
material  which  has  been  removed,  the  hundreds  if  not  thousands  of 
feet  of  terranes  which  have  slowly  been  taken  away,  we  readily  realize 
that  this  deposit  is  merely  an  insignificant  part,  delayed  in  transition , 
of  a long  and  slowly  creeping  train  of  land  drift  urged  onward  by  the 
present  drainage.  A closer  examination  of  the  drift  itself  corroborates 
this  view.  Over  extensive  flats,  where  erosion  is  weak,  the  surface  is 
covered  by  sand  and  silt,  which  probably  accumulates  faster  than  it  can 
be  carried  away.  But  where  the  land  has  a perceptible  slope  pebbles 
and  larger  fragments  appear.  As  the  slope  increases,  the  size  of  the 
fragments  also  increase,  showing  a control  by  sheet  flood  erosion. 
There  are  all  kinds  of  mixtures  of  rounded  and  angular  fragments. 
Occasionally  there  are  true  creek  gravels  far  away  from  the  present 
streams,  showing  that  these  have  changed  their  courses  in  the  past, 
adjusting  themselves  to  the  ground  structure.  The  material  is  all  of 
local  derivation,  as  may  be  seen  from  the  table,  given  below,  which 
fairly  represents  the  composition  of  the  coarse  ingredients  of  the  land 
drift,  from  which  it  is  largely  derived. 


Silver  Mine  District. 


41 


TABLE  SHOWING  PERCENTAGES  OF  DIFFERENT  KINDS  OF  PEBBLES  AND 
BOULDERS  IN  THE  ALLUVIAL  GRAVELS  OF  CIBOLO  CREEK,  NEAR 
CIBOLO  RANCH,  SHAFTER,  TEXAS. 


Kinds  of  Rocks. 

Diameter  of  pebbles  and  boulders 
in  inches. 

% 

inch. 

1 

inch. 

3 

inches. 

9 

inches. 

Andesite 

21 

31 

47 

50 

Undetermined  crypto-crystalline  volcanic  rocks 

26 

32 

24 

8 

Undetermined  volcanic  rocks  of  laminated  structure.. 

28 

17 

12 

16 

Undetermined  siliceous  volcanic  rocks 

9 

11 

11 

17 

Volcanic  tuffs 

7 

5 

Vesicular  lava 

7 

6 

Granite 

2 

2 

2 

Dark  shale 

7 

2 

1 

THE  RIO  GRANDE  DRIFT. 

As  we  approach  the  fiats  of  the  Rio  Grande  the  bed-rock  everywhere 
disappears  under  a drift,  which  at  first  seems  to  be  a continuation  of  the 
land  drift,  but  which,  as  we  come  near  to  the  great  water  way,  becomes 
mingled  with  material  of  distant  origin.  Metamorphic  rocks,  such  as 
schists  and  quartzites,  begin  to  appear,  and  there  is  a greater  abundance 
of  granites  among  the  larger  boulders.  Still  farther  out  this  becomes 
intermingled  with  layers  of  finer  alluvial  material  which  has  been 
deposited  by  that  stream  before  it  had  cut  its  changing  channel  down 
to  its  present  depth.  All  of  this  material  may  properly  be  called  the 
Rio  Grande  drift.  For  comparison  a table  is  here  appended  showing 
the  character  of  the  coarse  ingredients  in  the  present  alluvial  deposits 
of  the  river.  This  differs  somewhat  from  that  in  the  river  flats, 
especially  some  distance  away  from  the  stream.  The  most  notable  dif- 
ference is  the  absence  of  limestone  pebbles.  In  the  highest  and  oldest 
drift  the  limestones  have  been  entirely  dissolved  away  by  solution 
through  the  agency  of  the  ground  water. 


42 


Geology  of  the  Shafter 


TABLE  SHOWING  PERCENTAGES  OF  DIFFERENT  KINDS  OF  PEBBLES  AND 
BOULDERS  IN  THE  ALLUVIAL  GRAVELS  OF  THE  RIO  GRANDE 
RIVER.,  NEAR  PRESIDIO,  TEXAS. 


Kinds  of  Rocks. 

Diameter  of  pebbles  and  boulders 
in  inches. 

X 

inch. 

1 

inch. 

3 

inches. 

9 

inches. 

71 

63 

79 

50 

Vesicular  lava 

1 

18 

Vein  quartz 

4 

5 

1 

'Granite 

4 

1 

1 

5 

"Schists 

4 

Quartzite  

6 

6 

7 

21 

Sandstone  and  conglomerate 

2 

CJhert 

2 

2 

1 

Limestone 

13 

28 

11 

THE  IGNEOUS  ROCKS. 

The  igneous  rocks  of  the  Shafter  area  cover  about  one-third  of  its 
entire  surface.  As  to  their  mode  of  occurrence  they  present  all  the 
three  forms  in  which  igneous  rocks  appear.  1.  Lava  flows  and  tuffs. 
2.  Dikes  and  intrusive  sheets.  3.  Laccolitic  bosses.  It  is  not 
intended  to  discuss  here  the  petrographic  features  of  these  terranes,  for 
this  is  a task  that  must  be  left  to  the  specialist  in  that  line.  The 
purpose  is  merely  to  present  a brief  account  of  my  observations  of  these 
rocks  as  they  appeared  in  the  field. 

Laccolitic  Bosses. — Laccolitic  intrusions  probably  underlie  the  latter 
lava  flows  in  the  Chinati  mountains,  occupying  the  region  which  is  cen- 
tral to  the  crescent  that  marks  the  outcrops  of  the  sedimentary  rocks, 
but  they  are  now  mostly  concealed  by  the  later  extrusives  and  by  the 
land  drift.  To  all  appearances  they  were  submitted  to  extensive 
erosion,  and  cut  down  to  the  same  general  level  as  the  sediments,  before 
the  land  was  covered  by  the  last  great  lava  flows.  We  find  them 
exposed  now  principally  only  in  two  places. 

In  the  San  Antonio  canon  and  in  the  pass  which  separates  the 
Chinati  peak  from  the  south  Chinati  mountains  a dark  gray  diorite 
forms  the  lowermost  terrane.  This  is  of  a moderately  coarse  texture  and 
is  frequently  traversed  by  some  segregation  veins  measuring  from  an 
inch  to  several  feet  in  width.  These  consist  of  a red  granite  rock, 
which  is  more  acidic  than  the  diabase  and  also  somewhat  more  resistant 


Silver  Mine  District. 


43 


to  the  disintegrating  agencies.  It  frequently  forms  dike-like  ridges  on 
the  surface  of  the  country  rock  and  these  ridges  have  an  east  and  west 
trend. 

To  the  north  and  east  of  Ojo  Bonito  there  is  an  area  underlaid  by  a 
red,  coarsely  crystalline  granite,  possibly  also  of  a laccolitic  nature.  It 
extends  for  a distance  of  about  three  miles  from  northwest  to  south- 
east. It  has  a width  of  about  a mile  at  the  north,  and  tapers  almost  to 
a point  at  its  southern  end.  It  weathers  into  small  rounded  knobs  that 
appear  like  huge  boulders  or  small  hills  rising  above  the  land  drift. 

The  Cienega  Mountains  consist  of  a laccolitic  dome  of  a highly  acidic 
intrusive  of  light  color,  but  its  real  relations  in  the  field  are  not  known 
to  the  present  author. 

Dikes  and  Intrusive  Sheets  frequently  appear  in  the  sedimentary 
rocks,  running  in  a variety  of  directions.  One  of  these  dikes  is  seen 
on  a ridge  about  a quarter  of  a mile  west-northwest  from  the  Presidio 
Mining  Company’s  mill.  It  is  fifteen  feet  wide  and  trends  N.  86°  E. 
Another,  which  is  about  twelve  feet  wide  and  bears  N.  26°  W.,  cuts  the 
Shatter  beds  about  500  feet  north  of  the  town  of  Shatter  on  the  north 
side  of  Cibolo  creek.  About  a half  mile  to  the  west-northwest  of  the 
new  shaft  of  the  Presidio  mill  a dike  four  feet  wide  trends  E.  18°  FT. 
(See  Fig.  1.)  In  the  basin  of  Merita  creek  three  miles  southeast  of 
Shatter  another  dike  cuts  one  of  the  remnants  of  the  lava  flows  which 
have  been  carved  into  buttes  in  that  direction.  In  most  of  these 
instances  the  rock  is  of  a.  dark  basic  kind,  which  easily  disintegrates 
and  seldom  appears  on  the  surface.  In  the  Presidio  mine  two  of  these 
dikes  have  been  encountered  and  even  four  hundred  feet  below  the  sur- 
face they  have  been  found  to  be  thoroughly  disintegrated  and  changed 
into  a yellow  clay.  The  miners  call  them  “clay  dikes.” 

Intrusive  sheets,  which  follow  the  bedding  planes  of  the  sedimentary 
rocks,  are  hardly  less  frequent.  One  comes  into  view  at  the  base  of 
the  east  bluff  near  the  junction  of  the  Cibolo  and  the  Sierra  Alta  creek, 
and  several  lie  intercalated  with  the  Carboniferous  and  the  Cretaceous 
strata  on  the  south  and  the  southeast  side  of  the  Chinati  mountains. 
Some  of  these  are  of  a dark  gray  andesite  and  are  usually  quite  fresh 
and  unaltered  at  the  surface.  Some  consist  of  a light  gray  rhyolite. 
One  of  the  latter,  which  outcrops  in  an  arroyo  about  two  miles  north- 
west from  the  shaft  of  the  Presidio  Mining  Company,  has  a rough 
columnar  structure. 

As  we  approach  the  Chinati  mountains  from  the  east  the  dikes  and 
the  intrusive  sheets  become  more  and  more  frequent  and  at  the  same 
time  heavier.  Occasionally  they  thicken  into  true  laccolites.  On  the 
south  side  some  of  the  ridges  in  the  area  of  the  outcrops  of  the  Carbon- 
iferous limestone  consist  of  heavy  volcanic  intrusive  sheets,  which  lie 
in  the  same  position  as  the  stratified  rocks,  and  next  to  the  mountain 
the  sedimentary  rocks  and  the  intrusives  are  intermingled  almost  pro- 
miscuously. Evidently  the  source  of  these  intrusives  lies  in  that  direc- 
tion. 

Lavas  and  Tuffs. — Fully  one-half  of  the  land  is  yet  covered  by  lava 
flows,  which  no  doubt  have  extended  over  most,  if  not  all,  of  its  surface 
at  an  earlier  date.  They  have  since  been  carried  away  by  erosion. 
We  thus  find  them  mostly  absent  along  the  principal  drainage  lines, 
as  in  the  direction  toward  the  Rio  Grande  and  along  Cibolo  creek. 


44 


Geology  of  the  Shafter 


But  they  are  invariably  present  on  all  high  peaks  and  mesas.  These 
have  been  carved  out  usually  from  several  superimposed  flows,  whose 
upper  surfaces  are  apt  to  form  stair-like  terraces  on  their  sides. 
The  most  extensive  remnants  of  lava  flows  are  those  which  form 
the  body  of  the  Chinati  mountains,  and  whose  combined  thickness 
approaches  4000  feet.  In  all  probability  the  lavas  in  the  hills 
east  of  Shafter  are  remnants  of  the  same  flows  that  form  the  Chinati 
mountains  and  which  once  were  continuous  across  the  Cibolo  valley. 
In  texture  the  lavas  vary  from  coarse  and  fine  porphyries  to  vesicular 
and  almost  glassy  lavas.  In  the  mesa  known  as  the  Sierra  Alta,  which 
is  about  six  miles  north  of  Shafter,  the  lava  sheet  which  forms  the  cap 
is  underlaid  by  several  strata  of  tuffs  or  volcanic  sand,  some  of  a yellow 
and  gray  and  some  of  an  almost  pure  white  color.  The  flatter  kind 
appears  on  the  southeast  side  and  shows  a thickness  of  some  forty 
feet. 

The  vents  from  which  these  lavas  came  are  to  be  found  in  the  Chinati 
mountains. 

STRUCTURE. 

The  Chinati  Uplift. — The  controlling  structural  feature  is  an  east- 
ern half  of  the  base  of  a great  dome,  whose  west  side  lies  concealed 
under  the  Rio  Grande  drift  and  whose  north  side  extends  beyond  the 
limits  of  this  district.  (See  Fig.  6.)  The  diameter  of  this  uplift 
approaches  some  fifteen  miles,  judging  by  the  curve  of  the  strike  of  the 
tilted  sediments.  Its  center  lies  on  the  north  side  of  the  south  half  of 
the  Chinati  mountains.  The  sedimentary  rocks  dip  away  from  this 
point  in  all  directions,  with  more  or  less  regularity.  The  amount  of 
the  uplift  equals  at  least  the  entire  thickness  of  all  the  sediments 
exposedj  which  is  about  7000  feet,  so  that  if  these  had  not  been  removed 
they  would  now  form  an  elevation  seven  thousand  feet  high,  extending 
for  twelve  miles  east  and  west  and  north  and  south.  But  the  work  of 
erosion  has  long  ago  cut  down  to  the  base  of  this  dome  and  reduced  it 
to  the  general  level  of  the  surrounding  country.  This  has  exposed  the 
edges  of  the  sediments,  which  lie  in  a crescent  on  the  east  side.  Within 
this  crescent,  which  probably  forms  a continuous  ring  now  partly  con- 
cealed by  igneous  flows  and  by  drift,  the  erosive  forces  have  probably 
also  cut  down  the  central  core  of  the  igneous  rocks  that  caused  the 
uplift.  But  to  what  extent  this  central  igneous  mass  rose  above  the 
general  level  of  the  land,  as  it  now  stands,  and  to  what  extent  the  mass 
itself  may  have  been  cut  down,  it  is  not  possible  to  say.  It  may  have 
had  a considerable  elevation.  The  diorite  in  the  San  Antonio  canon 
and  the  granite  near  the  Ojo  Bonito  is  all  which  at  present  comes  up  to 
our  view. 


Fig.  6.— General  structure  of  the  Shatter  area  along  a line  from  northwest  to  southeast  from  San  Antonio  canon  to  south  of  Morita  basin.  A,  Lava  flows;  B,  Deep 

intrusive  diorite;  O,  Dike;  D,  Sedimentary  rocks.  1— Ohinati  uplift.  2— Morita  uplift. 


46 


Geology  of  the  Shafter 


On  top  of  the  uneven  plateau  of  this  reduced  and  leveled  dome,  the 
south  half  of  the  Chinati  mountains  as  we  find  them  now,  are  built  up 
from  several  superimposed  flows  of  extrusive  igneous  rocks.  These 
flows  show  no  great  slope  in  any  direction  and  the  molten  lavas  must 
have  had  a high  degree  of  fluidity. 

The  Morita  Uplift. — In  the  basin  of  Morita  creek,  about  six  miles  to 
the  southeast  of  Shafter,  is  another  small  and  probably  circular  uplift 
where  the  older  sediments  are  raised  to  the  level  of  the  present  surface. 
(See  Fig.  6.)  From  the  center  of  this  minor  uplift  the  sediments  like- 
wise dip  away,  in  all  directions  where  they  come  into  view.  On  its 
south  side  the  sediments  are  tilted  to  an  exceptionally  high  angle,  in 
one  place  nearly  70°.  The  north  and  greater  part  of  this  uplift  is 
covered  by  heavy  lava  sheets. 

Dips. — These  leading  features  of  the  structure  of  this  region  are 
shown  in  the  generalized  section  which  extends  from  the  San  Antonio 
canon  at  the  northwest  to  beyond  the  Morita  basin  at  the  southeast. 
Returning  to  examine  the  details  of  the  peripheral  part  of  the  principal 
uplift  we  find  that  while  the  Chinati  dome  in  a general  way  is  quite 
symmetric,  the  dip  of  the  uplifted  strata  frequently  shows  minor  varia- 
tions in  direction  and  in  amount.  Thus  on  the  south  side  of  the  Chi- 
nati mountains,  dips  ranging  from  18°  to  40°  were  measured,  the  aver- 
age being  perhaps  25°  or  30°  in  the  ledges  of  the  Chinati  series. 
Toward  the  southeast,  in  the  direction  of  the  Presidio  mine,  the  dip  is 
less  and  possibly  does  not  average  much  above  20°.  (See  Plate  1.) 
Farther  north  on  the  east  side  some  dips  were  noted  as  high  as  48°  and 
the  average  there  is  probably  not  less  than  35°.  Around  the  granite 
area  near  Ojo  Bonito  the  strike  changes  frequently  and  the  dip  is  very 
irregular  and  shows  a local  tendency  to  arrange  itself  radially  to  the 
granite.  This  suggests  that  the  granite  lies  at  the  center  of  a minor 
uplift  near  the  margin  of  the  larger  one,  and  that  the  granite  itself  is 
an  intrusion  which  caused  a minor  uplift. 

Another  structural  feature  is  clearly  shown  on  all  sides:  the  highest 
dips  are  nearest  the  center  of  the  uplift  and  the  lowest  dips  occur 
furthest  out.  This  has  a bearing  on  the  question  of  the  nature  of  the 
unconformity  between  the  Chinati  formation  and  the  Cretaceous  rocks. 
The  latter  seldom  have  a pitch  quite  as  high  as  the  former,  but  this 
circumstance  is  believed  to  depend  mainly  on  the  fact  that  the  Cre- 
taceous rocks  are  now  exposed  farther  out  from  the  main  uplift.  West 
and  north  of  Shafter  their  east  dip  is  mostly  less  than  15°  and  often 
no  more  than  10°,  or  even  less  than  this.  It  is  believed  that  the 
greater  tilting  of  the  Chinati  series  around  the  Chinati  uplift  is  not  to 
any  great  extent  the  result  of  pre- Cretaceous  displacement,  but  is  chiefly 
due  to  the  fact  that  the  most  highly  tilted  part  of  the  rocks  of  the  later 
age  has  been  eroded  away  and  is  not  now  to  be  seen.  Nevertheless  it 
would  evidently  be  wrong  to  infer  that  the  unconformity  between  the 
two  series  is  altogether  an  unconformity  without  dip.  No  doubt  there 
is  a difference  in  the  attitude  of  the  strata  of  the  two  ages,  hut  it  can 
not  be  asserted  that  this  difference  has  any  constant  relation  to  the 
Chinati  uplift.  Nor  does  it  appear  to  be  sufficiently  marked  to  be 
clearly  indicated  in  any  exposure  within  the  limits  of  this  area.  In 
places  where  the  Cretaceous  strata  lie  near  to  the  central  uplift,  their 
pitch  is  just  as  high  as  that  of  the  older  rocks. 


Pig.  7. — Illustration  of  faulting  tangential  to  an  uplift  in  the  peripheral  region,  caused  by  sinking  in  of  the  center  due  to  the  greater  weight  of  the  higher  laud. 

A . normal  faults. 


48 


Geology  of  the  Shafter 


Tangential  Faults. — The  peripheral  region  of  the  uplift  is  marked 
by  some  fault-lines  which  run  more  or  less  in  a tangential  direction 
relative  to  the  circle  of  the  dome-like  structure.  Near  the  location 
known  as  the  Humphrey  mine  two  such  faults  run  parallel,  east  and 
west,  only  150  feet  apart  for  a distance  of  about  a half  mile.  (See  Fig. 
11.)  They  hade*  slightly  to  the  north  and  follow  the  two  sides  of  a 
block  of  limestone  which  forms  a ridge  between  them,  extending  in  the 
same  direction.  This  limestone  is  the  same  that  caps  a high  ridge  to 
the  south  from  this  place,  and  it  has  been  lowered  not  less  than  a thou- 
sand feet.  The  faults  are  normal  and  the  downthrow  is  to  the 
north.  In  the  hills  to  the  south  and  west  from  this  place  some 
nearly  vertical  dikes  and  fissures  trend  in  the  same  direction.  One  such 
dike  is  near  the  diggings  locally  known  as  the  “Iron  Works.”  This  is 
a wide  dike  and  it  runs  east  and  west.  Again,  the  fissure  which  has 
been  explored  by  the  Chinati  Mining  Company  trends  about  10°  north 
of  east.  West  of  the  hill  on  which  the  first  diggings  in  the  Presidio 
mine  were  made,  there  is  a fault  running  about  N.  15°  E.  This  hades 
to  the  west  and  has  a downthrow  in  that  direction  of  nearly  three  hun- 
dred feet.  (See  Fig.  10.)  About  three-fourths  of  a mile  farther  north 
this  fault  changes  into  a fold,  a small  anticline,  which  can  be  traced  in 
the  arroyos  in  that  direction.  In  the  country  west  of  the  Sierra  Alta 
still  another  fault  was  seen  with  the  downthrow  to  the  west. 

In  all  of  these  instances  it  will  be  noticed  that  the  downthrow  of  the 
faults  is  toward  the  center  of  the  uplift,  while  the  dip  of  the  terranes  is 
in  the  opposite  direction.  As  volcanic  agencies  are  more  or  less  spas- 
modic when  compared  with  the  steady  and  unceasing  progress  of 
degradation,  it  is  reasonable  to  suppose  that  the  Chinati  uplift  rose  to 
its  tallest  height  in  the  earlier  stage  of  its  existence.  At  first  the  vol- 
canic forces  raised  the  dome  more  rapidly  than  the  work  of  erosion  cut 
it  down,  as  is  usually  the  case.  It  is  evident  that  during  all  the  fol- 
lowing time  when  the  elevated  tract  was  being  reduced,  it  must  have 
had  a greater  elevation  than  the  surrounding  country.  This  condition 
of  things  would  account  for  the  nature  of  some  of  the  faulting  just 
described.  The  central  area  being  the  higher  would  also  be  the  heavier 
and  some  sinking  would  naturally  take  place  there.  By  reference  to 
Fig.  7 the  above  explanation  of  this  kind  of  faulting  will  be  made  more 
clear.  It  is  a natural  result  of  a slow  subsequent  settling,  that  may 
be  looked  for  near  the  periphery  of  any  considerable  elevation,  however 
formed.  But  some  of  the  tangential  faults  seem  to  be  the  result  of 
another  process.  It  is  evident  that  if  the  inner  side  of  a block  lying 
just  outside  the  uplift  is  elevated,  it  is  apt  to  a certain  extent  to  be 
rotated  in  such  a way  that  its  outer  side  will  be  lowered.  Some  of  the 
dislocations  seen  in  Plate  1 were  probably  caused  this  way. 

*Hade,  deviation  from  the  vertical.  See  note  on  page  50. 


50 


Geology  of  the  Shatter 


Radiating  Faults. — There  are  some  small  faults  which  follow  a course 
that  is  more  or  less  radial  to  the  principal  uplift.  Thus  we  find  in  the 
west  bluff  of  the  Cibolo,  about  a mile  below  Shatter,  a block  in  the 
Shatter  beds  that  has  been  elevated  about  a hundred  feet.  It  is  marked 
off  by  a fault  on  its  north  side  running  W.  5°  1ST.  and  by  a broken  mono- 
cline on  the  south  side  trending  in  the  same  direction.  (See  Fig.  9.) 
Likewise  in  the  east  bank  of  the  Cibolo  looking  east  from  Shatter,  one 
sees  a number  of  these  small  faults  cutting  the  marls  and  limestone  at 
intervals  of  a few  rods,  each  with  a hade  to  the  south,  and  with  the 
pitch  in  the  same  direction.  The  downthrow  here  in  no  case  exceeds 
thirty  feet.  (See  Fig.  10.) 

Note — The  word  “hade”  (a  contracted  form  of  heald,  or  heeld)  means  the  in- 
clination of  a vein  from  the  vertical.  It  is  synonymous  with  underlay  and  is  the 
complement  of  the  dip,  which  signifies  the  deviation  from  the  horizontal.  The 
dip,  and  consequently  the  hade,  is  measured  at  right  angles  to  the  strike,  or  com- 
pass bearing. 


Fig.  9.— An  elevated  block  Of  the  Shatter  beds  seen  in  the  west  bluff  of  Cibolo  creek  a mile  below  Shatter.  A,  a fractured  fold;  B,  a fault;  C and  D,  two  fissures 

without  displacement. 


52 


Geology  of  the  Shafter 


SECONDARY  CHANGES  IN  SOME  ROCKS. 

Igneous  Metamorphism. — The  sedimentary  rocks  next  to  the  dikes  and 
intrusive  sheets  which  cut  through  them  are  sometimes  slightly  indu- 
rated from  the  contact  with  the  fused  mass  of  the  intrusive.  But  the 
change  produced  is  never  very  great.  Even  in  the  case  of  the  thickest 
dikes  the  effect  of  the  heating  is  not  perceptible  farther  out  than  four 
or  five  feet  from  the  intrusive.  In  no  instance  has  any  crystallization 
been  effected.  Very  often  in  case  of  the  thinnest  dikes  no  baking  effect 
whatever  is  visible.  Near  the  intrusive  granite  northwest  of  Sierra 
Alta  the  case  is  different.  A limestone  which  lies  in  contact  with  the 
granite  has  been  changed  to  a true  marble  of  moderately  coarse  crystal- 
line texture.  In  places  the  marble  is  snowy  white  and  elsewhere  it  is 
traversed  by  wavy  dark  streaks.  The  limestone  which  belongs  to  the 
Cieneguita  beds,  contains  a small  admixture  of  coarse  sand,,* and  these 
grains  have  been  fused  into  new  forms,  different  from  the  sand  in  the 
unaltered  rock.  The  metamorphism  has  affected  the  limestone  through 
a thickness  of  from  twenty  to  thirty  feet  and  the  marble  ledge  is 
exposed  for  a distance  of  at  least  a thousand  feet. 

Changes  Effected  by  Ground  Moisture. — The  changes  wrought  by 
igneous  metamorphism  in  this  region  are  on  the  whole  not  as  important 
as  the  effects  produced  by  the  action  of  ground  water.  These  are 
almost  everywhere  apparent.  The  ground  water  has  frequently  cut 
caverns  in  the  limestone  and  again  filled  these  with  various  mineral 
and  mechanical  deposits.  In  the  underground  work  of  the  Presidio 
mine  several  empty  caverns  have  been  explored,  and  it  is  beyond  ques- 
tion that  some  of  the  ore  deposits  are  fillings  in  such  ancient  caverns 
in  the  carboniferous  limestone.  In  the  base  of  the  Cibolo  limestone, 
three  miles  north  of  Shafter,  there  is  a brecciated  deposit  which  fills  an 
old  collapsed  cavern.  It  consists  of  fragments  of  all  sizes,  some  angular 
and  some  water-worn,  blocks  of  shale  from  the  lower  part  of  the 
Chinati  series,  cherts  from  the  upper  beds,  pieces  of  glassy  and  spheru- 
litic  lavas,  and  some  other  rocks  not  known  in  outcrops  near  by. 

The  carboniferous  limestone  is  quite  free  from  impurities  and  has 
been  more  readily  dissolved  by  the  ground  water  than  any  other  forma- 
tion. Small  fissure-like  passages  were  noted  in  many  places  on  the  sur- 
face, but  they  were  sometimes  collapsed  and  sometimes  filled  with  clayey 
or  ochreous  deposits,  or  with  pure  and  crystalline  calcite.  In  the  lime- 
stone wedge  near  the  Humphrey’s  mine  there  was  a solid  body  of  cal- 
cite three  feet  in  diameter  which  had  been  followed  to  a depth  of  sev- 
eral feet  in  one  of  the  prospect  holes.  The  upper  part  of  the  fissure 
explored  in  the  Chinati  mine  has  clearly  been  opened  up  into  cavernous 
expansions  by  the  ground  water,  and  these  were  later  filled  by  the  ore 
bodies. 

Another  effect  produced  by  the  ground  water  consists  in  a silicifica- 
tion  of  the  limestones.  In  some  places  this  silicification  ceased  in  its 
incipiency  and  the  silica  has  been  deposited  chiefly  in  the  organic  bodies 
which  the  limestones  contain.  The  fossils  which  such  ledges  carry  fre- 
quently weather  out  conspicuously  on  the  surface.  This  is  the  condi- 
tion of  the  thin-bedded  member  of  the  Cibolo  beds  north  of  Shafter. 
Elsewhere  the  body  of  the  rock  is  also  affected  and  the  entire  mass  is 


Silver  Mine  District. 


53 


charged  with  more  or  less  silica.  Such  is  often  the  Zone  of  Sponge- 
Spicules  in  the  Cibolo  beds.  South  of  the  Chinati  mountains  and 
north  of  the  diggings  already  referred  to  as  the  “Iron  Works”  some 
of  the  thin-hedded  limestones  have  been  altered  to  a wholly  siliceous 
white  chert  which  yet  perfectly  retains  the  characteristic  bedding  planes 
of  these  ledges.  The  thickened  lenses,  which  are  characteristic  of 
the  horizon,  are  unmistakably  present,  although  every  trace  of  fossil 
remains  seems  to  have  been  destroyed.  In  general  the  silicifica- 
tion  of  the  limestones  of  the  Chinati  series  is  more  pronounced 
on  the  south  side  of  the  mountains  than  to  the  east  and  north.  On 
the  surface  of  the  Cibolo  limestones  one  often  finds  rusty  patches 
impregnated  with  ferruginous  and  siliceous  material.  These  patches 
nearly  always  extend  down  into  the  bed-rock  along  irregular  fis- 
sures. Quite  often  they  seem  to  partake  of  the  nature  of  “chimneys,” 
in  that  they  go  down  more  or  less  vertically  without  following  any  of 
the  joints  in  the  rock.  Many  of  them  have  been  explored  by  prospect 
holes  and  found  to  contain  limonitic  siliceous  ore,  that  not  infrequently 
have  shown  a trace  of  silver  and  occasionally  of  other  minerals.  They 
are  most  frequent  about  the  diggings  on  the  hill  near  the  silver  mine 
and  some  were  the  surface  outcrops  of  the  ore  in  that  mine.  Some 
appear  on  the  bluff  of  the  Cibolo  limestone  which  follows  the  east  bank 
of  Sierra  Alta  creek.  About  a mile  and  a half  north  from  the  junction 
of  this  creek  with  the  Cibolo  there  are  two  pointed  buttes  rising  from 
the  lower  slope  of  this  bluff.  These  consist  of  a hard  and  dark  brown 
rock  of  fine  texture  which  in  the  smaller  butte  is  very  much  like  a fine 
sandstone,  but  which  in  the  larger  one  contain  minute  crystals  of  some 
feldspar.  On  the  north  side  of  this  butte  a layer  of  siliceous  tufa, 
clearly  a precipitate,  lies  in  close  contact  with  the  harder  rock  and 
apparently  changes  into  this  and  is  continuous  with  it.  I am  inclined 
to  believe  that  these  two  “necks”  are  related  to  the  rusty  impregnations 
in  the  limestone  elsewhere,  and  that  they  are  all  due  to  the  activity  of 
some  underground  water  which  has  followed  irregular  lines  of  fissure 
and  cavernous  passages  in  the  limestone.  When  we  consider  that  the 
igneous  intrusions  are  not  very  far  away  and  that  siliceous  tufa  usually 
is  a deposit  from  heated  waters,  the  suggestion  is  at  hand  that  the 
ground  moisture  in  this  case  was  in  a heated  state.  As  feldspar  has 
been  produced  artificially  in  a wet  way,  under  pressure  ,it  is  not  alto- 
gether impossible  that  the  rock  in  the  necks  is  an  underground  product 
of  a hot  spring,  now  laid  bare  by  erosion  of  the  containing  terrane. 

Even  the  igneous  rocks  show  the  effects  of  the  action  of  ground  water. 
Most  of  the  dikes  have  been  thoroughly  disintegrated  and  in  many 
places  where  explored  in  excavations  they  have  been  found  to  be  reduced 
to  a yellow  soft  clay.  In  the  Presidio  mine  the  rock  in  one  of  these 
dikes  is  altered  to  a pure  white  and  structureless  kaolin. 

In  the  diorite  near  the  Ellsworth  mine  in  San  Antonio  canon  there 
is  indubitable  evidence  of  the  action  of  heated  moisture  or  gases  in  a 
subterranean  fissure.  On  one  side  of  a dike-like  ledge  which  cuts  the 
diorite,  there  are  stalactitic  structures  of  a greenish  silicate  that  show 
all  degrees  of  irregularity  and  symmetric  perfection  of  form.  They 
are  from  one-half  to  one  and  one-half  inch  in  diameter  and  exhibit  thin 
concentric  laminae  as  well  as  a radial  structure  in  cross  section.  To  all 


54 


Geology  of  the  Shafter 


appearances  these  structures  are  related  to  the  stalactites  described  by 
E.  S.  Dana  from  Hawaii  and  which  he  found  had  been  made  by  super- 
heated vapors  in  underground  fissures. 

MINERAL  DEPOSITS. 

With  the  above  account  of  the  geology  of  the  district  a few  notes  on 
the  mineral  occurrences  will  perhaps  not  be  out  of  place. 

At  the  outset  it  may  be  stated  that  the  mineral  deposits  in  this  dis- 
trict are  limited  to  the  older  sediments  and  to  the  deep  intrusives.  The 
Cretaceous  rocks  and  the  latest  extrusives  have  suffered  comparatively 
little  change  from  the  mineralizing  processes.  They  sometimes  exhibit 
fissures  filled  with  calcite  and  occasionally  the  igneous  flows  have  been 
altered  to  a soft  green  chloritic  rock  or  are  otherwise  decomposed,  but 
they  are  not  known  to  contain  any  mineral  deposits  of  importance. 
Such  deposits  occur  here  in  five  forms,  viz. : as  lodes  resembling  fillings 
in  ancient  caverns,  as  impregnations  in  small  fissures  of  irregular  form, 
as  contact  deposits,  as  fissure  veins,  as  fault-fissures,  and  as  fissure 
zones. 

Lode . — To  the  first  one  of  these  categories  belong  the  ores  of  the 
Presidio  mine.  It  lies  in  irregular  pockets  or  lodes  from  a few  feet  to 
nearly  a hundred  feet  in  length.  These  are  connected  by  more  narrow 
leads,  where  the  ore  body  is  often  more  sharply  marked  off  from  the 
unaltered  ledges  of  the  country  rock.  (See  Plate  1.)  The  body  of  the 
ore  is  mostly  a yellow,  somewhat  porous  rock-like  mass  of  siliceous  mate- 
rial in  which  crystals  of  calcite  and  galena  are  not  seldom  seen.  In  the 
average  run,  there  is  only  about  3 per  cent  of  carbonate  of  lime  and 
from  45  to  50  per  cent  of  silica.  The  percentage  of  calcareous  material 
increases  toward  the  wall  of  the  ore  bodies  and  it  usually  also  increases 
with  the  amount  of  galena  which  is  present.  The  bulk  of  the  silver  is 
in  the  form  of  a chloride  dispersed  through  the  body  of  the  ore.  But 
there  are  also  pockets  of  galena  rich  in  silver.  This  is  sometimes 
altered  to  cerussite.  Sphalerite,  quartz  and  malachite  are  sometimes 
found  with  the  other  minerals.  While  the  Presidio  mine  is  the  prin- 
cipal instance  of  this  kind  of  ore  deposits,  other  lodes  of  a similar 
nature  have  been  found  in  the  same  limestone  to  the  west.  The  lodes 
occur  only  in  the  most  pure  strata  of  the  Cibolo  limestone,  and  in  the 
Presidio  mine  they  reach  lower  and  lower  levels  in  this  formation  as 
they  are  followed  to  the  east,  descending  more  rapidly  than  the  actual 
dip  of  the  sediments. 

Chimneys. — The  lodes  in  the  Presidio  mine  are  undoubtedly  geneti- 
cally related  to  the  siliceous  impregnations  which  frequently  appear  on 
the  outcrops  of  the  Carboniferous  rocks  and  which  have  already  been 
described  as  a result  of  the  activity  of  the  ground  water  at  an  earlier 
period.  None  of  these  chimneys  have  as  yet  proved  to  be  rich  enough 
to  be  profitably  worked  and  they  probably  mark  some  less  open  passage- 
ways of  the  solutions  which  deposited  the  ore  in  the  Chinati  mine. 
Even  some  of  these  are  clearly  fillings  in  old  caverns,  whose  etched 
walls  are  plainly  exposed  in  some  prospect  holes.  Galena,  calcite,  and 
hematite  occur  in  the  siliceous  mass  which  now  fills  them. 

Contacts. — At  the  contact  of  the  granite  with  the  Cieneguita  beds 


Silver  Mine  District. 


55 


near  Ojo  Bonito  masses  of  hematite  and  limonite  lie  against  the  side 
of  the  intrusive,  in  some  places  near  the  exposure  of  the  marble  pre- 
viously described.  Farther  north  some  pieces  of  float  of  crystalline 
magnetite  was  taken,  and  it  is  reported  that  larger  bodies  of  this 
mineral  lie  along  the  same  contact  beyond  the  limits  of  this  district. 
To  the  same  class  of  deposits  we  must  also  refer  the  hematite  and  the 
magnetite  masses  which  have  been  explored  on  the  south  side  of  the 
Chinati  mountains  at  the  diggings  known  as  the  “Iron  Works.”  These 
ore  bodies  are  perhaps  primarily  the  result  of  fusion  by  contact  with  the 
molten  mass  of  the  intrusives  at  each  place,  but  no  doubt  they  have  later 
been  altered  and  affected  by  solvent  agencies  following  the  zone  of  con- 
tact. 

About  one  mile  south  of  Spencer’s  spring,  on  the  south  side  of  the 
mountains,  two  explorations  have  been  made  on  another  contact  of 
igneous  intrusives.  The  uppermost  part  of  one  of  these  shows  a cavern- 
wall  which  is  covered  with  a stalactitic  calcareous  deposit.  Busty 
cavern  “box  work,”  such  as  is  seen  in  the  Great  Wind  cave  in  the  Black 
Hills,  covers  another  wall.  Limonite,  galena  and  cerussite  were  the 
principal  minerals  in  this  location  and  the  latter  occurs  in  a vein  in  the 
bed  of  an  arroyo  near  by  to  the  south. 

Fissures. — The  main  explorations  of  the  Chinati  mine,  which  is 
located  about  two  miles  west  of  the  Presidio  mine,  have  been  made  on 
a fissure  vein  bearing  E.  10°  N.  (See  Fig.  10.)  This  fissure  is  in  the 
Cibolo  limestone  and  its  hade  is  about  37°  to  the  north.  It  shows  on 
the  surface  as  a slight  sag,  running  several  hundred  feet.  About  a hun- 
dred feet  to  the  south  another  fissure  runs  on  a parallel  course  and  this 
has  'also  been  found  to  contain  some  ore.  Hear  the  surface  the  upper 
vein,  where  excavated,  shows  several  cavernous  expansions,  but  farther 
down  it  is  a well  defined  fissure  with  stringers  of  galena  and  calcite, 
mostly  next  the  hanging  wall.  It  has  been  followed  for  about  five  hun- 
dred feet  horizontally  at  three  levels  about  one  hundred  feet  apart. 


Fig.  10.— Section  at  t^e  Cftinatl  mine,  from  N,  tq  4.  mineralized  fissures;  B,  Presidio  beds;  0,  Cibolo  limestone  and  shales;  D,  Igneous  rocks  of  tfce  Chinatl  M^s. 


Geology  of  the  Shafter 


57 


In  the  hills  of  the  same  limestone,  from  one  to  two  miles  southwest 
of  Humphrey’s  location,  and  about  a half  mile  west  of  the  “Iron 
Works,”  are  three  small  fissure  veins  which  run  north  and  south.  These 
carry  carbonates  and  silicates  of  copper  in  the  form  of  chrysocolla, 
azurite,  and  malachite.  In  some  small  excavations  which  have  been 
made,  these  veins  show  a thickness  of  only  a few  inches.  One  hades  to 
the  east  and  another  to  the  west  at  an  angle  exceeding  45°  from  the 
vertical.  The  limestone  near  the  two  veins  which  are  farthest  to  the 
west,  has  been  changed  to  a marble. 

Fault-Fissures. — In  the  Humphrey  mine,  on  section  9,  some  four  hun- 
dred feet  of  tunneling  has  been  done  on  a fault  vein  which  follows  the 
north  side  of  a wedge  of  Cibolo  limestone  that  has  been  let  down  several 
hundred  feet  as  already  described.  The  ore  which  has  been  brought  up 
is  limonitic  and  siliceous  and  has  been  found  to  contain  lead  [galena 
and  cerussite]  as  well  as  silver.  The  quality  of  the  ore  is  believed  to  war- 
rant more  extensive  exploration.  This  ore  lode  has  in  one  place  been 
followed  into  a cavernous  extension  in  the  limestone  block  on  its  south 
side. 


F)g.  11.— Section  N — S,  at  the  Humphrey  mine.  A,  Humphrey  shafts  on  ai 


Silver  Mine  District. 


59 


Fissured  Zones. — The  Ellsworth  mine  in  the  San  Antonio  canon  is 
in  what  appears  to  be  a fissured  zone  which  cuts  the  diorite  already 
described,  and  also  the  overlying  lavas.  Some  rich  pockets  of  native 
silver  are  reported  from  this  claim  and  galena  ore  is  also  found.  The 
trend  of  the  Assuring  is  somewhat  south  of  east. 

SUMMARY  OF  GEOLOGICAL  EVENTS. 

The  earliest  event  recorded  in  this  locality  is  the  encroachment  of  the 
sea  during  the  later  part  of  the  Carboniferous  age  and  the  deposition 
in  the  marginal  region  of  this  sea  of  some  sandy  and  conglomeratic 
shales  and  limestones.  Floating  ice  possibly  brought  some  boulders  and 
pebbles  which  were  added  to  the  finer  sediments,  and  stray  fragments 
of  vegetation  sometimes  drifted  out  from  the  shore  and  became 
imbedded  in  the  accumulating  mud  of  the  Cieneguita  formation.  The 
bottom  of  the  sea  kept  on  sinking  and  the  shore  receded  until  littoral 
sediments  no  longer  were  brought  this  far  out  and  the  clearer  waters 
gave  a habitat  to  a deep-water  marine  fauna  of  cup-corals,  crinoids, 
foraminifera,  and  reticulate  sponges,  which  are  imbedded  in  the  Cibolo 
limestones.  This  condition  of  things  lasted  until  near  the  end  of  the 
Paleozoic,  at  the  time  of  the  Appalachian  revolution.  Then,  or  at  some 
time  during  the  early  Mesozoic,  the  bottom  of  the  sea  was  elevated  and 
became  land.  This  land  was  subjected  to  erosion  and  destruction  and 
hundreds  or  perhaps  thousands  of  feet  of  the  latest  sediments  were  again 
carried  away  to  other  seas.  In  this  locality  the  land  thus  carved  out 
from  the  earlier  sediments  had  reliefs  of  several  hundred  feet.  Pos- 
sibly there  was  also  some  folding  and  tilting  of  the  ground,  but  this 
can  not  have  been  very  extensive.  At  the  end  of  the  Jurassic  the  land 
again  sank  and  the  sea  advanced  once  more.  Over  the  hills  and  in  the 
valleys  of  the  Jurassic  lands  the  Cretaceous  sea  laid  down  first  lagoon 
muds,  beach  gravels,  and  sand,  and  later  on  more  open  sea  sediments, 
such  as  marly  silts,  limestones,  and  deep-sea  ooze.  This  sea  remained 
until  the  be, ginning  of  the  Tertiary  or  possibly  somewhat  later.  Then 
its  bottom  was  elevated  in  the  great  uplift  which  resulted  in  the  mak- 
ing of  the  Eocky  mountains.  This  region  lies  in  the  trend  of  that 
great  uplift  and  was  part  of  it.  The  Tertiary  age  was  a time  of  great 
volcanic  activity  and  the  deep  intrusives  of  the  Chinati  mountains  were 
now  forced  in  under  the  sedimentary  rocks  and  lifted  them  up  into  the 
Chinati  and  the  Morita  domes,  which  rose  high  above  the  rest  of  the 
land.  No  doubt  this  did  not  happen  all  at  once  hut  gradually  and  by 
repeated  intrusions.  In  the  meantime  the  elevations  suffered  continued 
degradation,  and  the  relief  of  the  land  was  reduced  until  the  uplifted 
domes  were  lowered  to  about  the  same  level  as  the  laud  around  them. 
Then  again  volcanic  disturbances  gained  the  ascendency.  They  had  by 
this  time  begun  to  reach  the  surface  and  they  poured  out  liquid  lavas. 
These  spread  over  the  land  in  thick  flows  that  must  have  extended  out 
for  perhaps  twenty  or  thirty  miles  from  the  vents  in  the  Chinati  moun- 
tains. This  activity  continued  for  a long  time,  but  at  last  it  came  to 
a standstill.  Probably  it  was  not  continued  far  into  the  Pleistocene 
age,  for  the  heavy  igneous  overflows  have  already  since  then  had  time 
themselves  to  be  extensively  invaded  by  the  destructive  elements.  The 


60 


Geology  of  the  Shafter 


most  extensive  erosion  of  the  earliest  glacial  deposits  in  the  Mississippi 
valley  dwindles  into  insignificance  when  compared  with  the  post-plntonic 
erosion  in  this  territory.  The  latest  volcanic  flows  have  contributed  the 
largest  quota  to  the  detrital  accumulations  of  the  land  drift.  This 
drift  is  the  most  recent  product  of  constructive  agencies  in  this  region. 

ACKNOWLEDGMENTS. 

The  author  wishes  to  express  his  obligations  to  Dr.  Wm.  B.  Phillips, 
the  Director  of  the  Survey,  for  much  help  and  advice  in  all  of  the  work 
done ; to  Mr.  B.  F.  Hill  for  aid  in  the  field ; to  Prof.  F.  W.  Sardeson  for 
aid  in  the  identification  of  some  bryozoa  and  corals ; to  Prof.  J.  W.  Beede 
for  examining  the  Paleozoic  pelecypods ; to  Dr.  T.  W.  Stanton  for  deter- 
mining some  fossils  of  the  Cretaceous  rocks ; to  Prof.  J as.  Perrin  Smith 
for  the  identification  of  two  cephalopods,  and  to  Dr.  G.  H.  Girty  for  ex- 
amining some  Carboniferous  fossils. 


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